Thank you June,
Now when we see lumps of this nature, we do have a few concerns. Since this just arose so quickly, our most likely suspects would be allergic reactions to an insect sting, abscesses (from a wound or foreign body he's not told you about), cysts (fluid filled growths), and hematomas (akin to a blood blister). These could all potentially appear without warning and do not necessarily have to be sore to the touch.
That said, with the location, attachment and firmness of this mass, we do have to also be wary of other more sinister types of masses. Especially since we can see a specific firm fast growing tumor that can arise aroung the shoulder blades called an sarcoma. And while all tumors have the ability to grow very quickly, this one in particular is known to do so and would therefore be a wee worry in this situation at this stage.
With all this in mind, I do have to say that the best way to approach an abnormal mass like this is to have the vet evaluate the mass via fine needle aspiration (FNA). This is where the vet uses a needle to harvest cells from the mass. Animals do tend to be quite tolerant of this and often the sampling can be done in the consult room without sedation. If the vet performs the FNA and removes pus, then its a sign of an abscess/infection (which can be treated with antibiotics). If clear fluid is removed, then it is likely a cyst (which can be drained and often only removed for cosmetic reasons). Otherwise, if these are not the culprit, then the vet will stain the cells extracted to identify the nature of the mass. As well, at the same time, the vet will be able to examine your Whiskey for other lumps or bumps and can evaluate the local lymph nodes to make sure they are not also enlarged.
Finally, I would just note that while you are considering and arranging having him seen, you can consider warm compressing this lump. It won't help if we have a mass or an abscess but it can reduce our allergic reaction and hematoma concerns. Therefore, this would be something to try tonight for Whisky. Just to note in case you don't have a warm compress on hand, you can make a microwaveable one by filling a sock 2/3rd full with rice and popping it in the microwave for a few minutes (of course, do give it a shake afterwards to distribute the heat and make sure its not too warm before use). If you try this and it settles, we can all breathe a wee sigh of relief. If it doesn't, then we'd want to have this checked for him.
Overall, with this being a firm mass on the back of the neck (and therefore a risk of something worrying), I must say that it would be prudent to have your vet check this lump sooner rather than later. If he is due for a booster soon, consider moving that appointment up to get him seen. Because the sooner you know the identity of this abnormal growth, the sooner you will know if it needs to be removed. (Or you can delay as long as it doesn't change in size while you monitor it). If it is found to be benign, then its no worry. But if is is found to be cancerous, then this will put you in the best position to address it and remove it for him. And as I am sure you can appreciate, early intervention means he is still young enough to be a good anesthetic candidate as well as early removal of a mass gives you the best chance of removing it all, preventing it getting to large to remove, preventing it spreading into the body, and giving you the best chance of settling this for him.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,