Hi again Jamie,
From your history, it sounds like a lot of stressors bombarding Milo just now. And if he was stressed by the noise and now there is a puppy, all of this can be very upsetting for him. So, we need to get that Feliway perhaps even see if you can get a small refill from the pet store while waiting for this other one, so we can help him faster. And I would note that you are right to keep him indoors, since he could be a flight risk here.
Otherwise, in this situation, the main focus needs to be encouraging him to eat and drink properly. This may mean sitting with him during meal time to help him feel safe. Otherwise, you can tempt him with favorites or light diet options (ie boiled chicken, white fish, scrambled egg, or meat baby food -as long as garlic/onion free). And if we just cannot get a good volume of food into him, we can cheat by using a nutrient dense diet (ie Hills A/D, Royal Canin Recovery, Clinicare liquid diet) or even kitten food so we get more nutrition into him per bite he will take and thus help keep his weight on. Otherwise, we could consider syringe feeding any of those diets (or watered down kitten food) to just keep the nutrition going in, avoid any weight loss or hepatic lipidosis (a health worry when cats aren't eating properly), and keep him going until the Feliway arrives and he starts to cope with this puppy situation.
Finally, in regards ***** ***** him cope with this stress, further to Feliway, you may also want to consider trying him with some alternatives. These can even be used alongside the Feliway. One of these is a diet called Calm (by Royal Canin). This contains a number of supplements that have been found to provide stress relief to cats. As well, there are nutritional supplements like Zylkene and Kalmaid which are over the counter at vets and pet stores can help soothe anxious cats. As well, there is even a Bach's flower remedy for stressed kitties that could be used.
Overall, your lad 's signs tell us that he is very stressed and he is feeling vulnerable. Therefore, we need to consider getting some Feliway now and/or the above de-stressing treatments to help him cope but most importantly we need to take a more aggressive approach to getting nutrition into Milo. So, try tempting, using more nutrient dense foods to offset his reduced appetite, or even syringe feeding food to help prevent him from losing weight, developing hepatic lipidosis (a complication when cats eat poorly), and help him cope with this situation.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,