I am sorry to hear that your pup Archie is miserably itchy and scratches/licks his groin area enough that it is now red.
Allergies are the most common cause of itchy skin and can give him a skin crawling, all over itchy type effect which many dogs find irritating. Westies as a breed have a tendency to suffer from allergies, and are predisposed to develop secondary skin infections when they lick and chew their itchy skin. I'll give you an allergy rundown of likely causes for skin troubles. He may have more than one allergy given how symptomatic he is now. Dogs that have one allergy often develop several with time. The effect of multiple allergies aren't additive, they actually compound one another.
Even if you don't see fleas I do recommend using protection. Flea bite allergy is the most common allergen and it only takes one bite a month to make an allergic dog scratch so I recommend using flea prevention even if you never see one again. Frontline Plus, Advantage II or Advantix are excellent as is the Seresto Flea collar. I don't normally recommend flea collars, but this one really works and lasts for at least 6-8 months. New prescription products on the market such as Bravecto or Nexgard, which tend to have less flea population resistance because they are new, may be worth trying too. Then only problem with pills is that the flea needs to bite your dog and take a blood meal to die, so allergic dogs will still react. Don’t use over the counter products, especially Hartz or Sargents, as most are ineffective if not toxic. Unfortunately even after the fleas are gone the allergic reaction can continue for weeks and I will discuss controlling that reaction below.
Other allergens can be inhaled (like grass pollen, dust mites or molds).
You can use a combination of antihistamines and high doses of omega-3 fatty acids to help with the symptoms of flea bite allergy and inhaled allergens. In combination fatty acids and antihistamines work synergistically, much better than either one alone. If his symptoms worsen seasonally I would think that inhaled allergens are a part of his problem.
You can try:
1)Benadryl (diphenhydramine only don't use the combination products with acetaminophen or decongestants as they can be toxic for dogs) at a dose of 1mg to 2mg per pound of body weight or one half of a 25mg capsule per 7 to 15 pounds of body weight orally every 8 hours.
OR 2)Claritin (loratidine) at 1/2 of a 5mg per 10-15 pounds of body weight once or twice daily.
OR 3)Hydroxyzine at 1mg per pound of body weight orally every 8 hours.
OR 4) Chlorpheniramine at 4mg to 8mg per dog once or twice daily.
OR 5) Zyrtec (Cetirizine hydrochloride) at 1/2 mg per pound of body weight orally every 24 hours. That would be one 10mg tablet per 20 pounds of body weight. Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because dogs cannot tolerate decongestants.
Some dogs do better on one antihistamine rather than another. Give the one you pick a 3-5 day trial and if he isn't less itchy try another. Be aware antihistamines can cause sleepiness or hyperactivity in some dogs. These side effects do wear off with repeated use.
Omega-3 fatty acids are fish oil products. Welactin is a good brand name product. Use the high end of the dosing schedule for your pup's weight. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give him 20mg of EPA per pound of body weight per day. For example a 13 pound dog could take 260mg of EPA per day.
Cool water baths with an oatmeal shampoo or chlorhexiderm shampoo (which is antibacterial and antifungal) and a conditioner with an antihistamine may help. The water rinses off allergens and the cool temperature soothes itchy skin. You should follow up any shampoo with a conditioner to add moisture back to his coat and skin. There are some with skin protectants and antihistamines which can be very, very soothing (for example EpiSoothe Cream Rinse or Vitasone® Leave on Conditioner Hydrocortisone 1% ). Do not bathe him for 2 to 3 days before or after applying flea control products or the bath will interfere with the product's efficacy.
Food allergy is very possible with him as well if he seems to be itchy all year round. Dogs can develop allergies to any protein or carbohydrate so even if he is only fed one food that can contain ingredients that he is allergic to. Dogs with food allergies tend to lick and scratch their paws, face and ears the most, but any of the "allergy reactive areas" can be affected.
Make sure that the food that you put him on is a true hypoallergenic diet. The trouble with "limited ingredient" or "low allergy" pet store brands is that the same machinery is used on multiple lots of food without sterilization cleaning in between. So for example even if a food says it has salmon and rice if the previous batch had beef and corn then you will get traces of those ingredients in your bag of food. Not a big deal if your dog isn't allergic but a waste of money thinking that the food was hypoallergenic and not good for your dog if those happen to be allergens for your dog. The veterinary brand true hypoallergenic foods are more expensive because it isn't cheap to thoroughly remove all traces of a previous food mixture from the machines used to process food.
Generally what I recommend is trying to clear the skin with a prescription hypoallergenic diet and then adding one food item (chicken, beef, corn wheat etc) every month to see what they react to. Then we can find a regular food to try that does not have any ingredients he is allergic to. As far as permanent diets I do tend to stick with Purina Pro Plan brands or Nature's Recipe as I find those rarely if ever have cross contamination. Purina Pro Plan Turkey and Barley or Nature's Recipe Vegetarian or Venison are pretty good products. I know that this isn't easy from personal experience (my last dog was allergic to wheat) and it is time consuming, but worth it.
If you choose to try testing/treating him for a food allergy I recommend that you try a true hypoallergenic diet like Hills z/d or Purina Pro Plan Veterinary Diets HA. No treats, flavored medication or bones while on the diet and it must be used for a least 12 to 16 weeks to see the full effects. You should see some improvement in 6 to 8 weeks.
Hydrocortisone ointment (for example Cortaid) applied to the very itchy places (as long as they aren't raw) twice daily may help as well if needed. Some sprays have alcohol which can be painful on raw skin and lead to more inflammation. I don't like oral steroids, although sometimes a short course is necessary, but a topical directed at the problem areas can be very helpful and are less likely to cause side effects.
Another option if the antihistamines and omega 3's aren't enough is a product called Atopica. It suppresses the immune system a bit so it decreases allergic symptoms but it doesn't have as many harmful side effects as systemic steroids.
Another option is a newer drug called Apoquel (generic name oclacitinib) which interferes with the allergic pathway. It works very quickly to stop the symptoms of an inhaled or flea allergy. Most dogs are reported to be much more comfortable in a day or so. It is not as helpful for food allergies as those affect a different allergic pathway.
If you are interested discuss these medications with your veterinarian as they are prescription products.
Another option if you are interested, is trying immunotherapy. He would need to be tested to determine exactly what he is allergic to, and then he is given small amounts of the allergen to build up his tolerance to it, increasing the amount of allergen in the injection incrementally so that his immune system no longer responds to it. This isn't a quick fix, it takes time to slowly build up their tolerance and as he develops new allergens things may need to be added, but it is an option.
There is a new injection out (Cytopoint) that can be given by your veterinarian that can help with allergies for 4-8 weeks. It neutralizes a key allergic pathway inflammatory protein, thus stopping the itch before it starts.
If it's been a while since his last exam parasites like cheyletiella, demodex or sarcoptes mites, or a secondary bacterial or fungal skin infection should be looked for by your veterinarian as well if he isn't improving as they can lead to very itchy skin.
It has been my pleasure to help you today, and I hope that I have earned my 5 star rating. Please remember to rate my service by selecting the 5 stars at the top of the screen (rating me now does not close your question). You are welcome to ask follow up questions about my response here until you are satisfied, simply use the reply box and let me know. Thank you!