There are quite a few people on
Etsy - Your place to buy and sell all things handmade, vintage, and supplies that make clothing for dogs that are made to fit. They will take your measurements and make what you need. I think they are really reasonably priced, too. I think there are a few YT people that sell things on Etsy.
Getting a better fit might help. Does he dislike wearing sweaters or is he just managing to get out of them because of the fit?