He will breathe a lot better if you take that stupid collar off of him. Nothing pisses me off more than ignorant owners putting collars on brachycephalic dogs.
Get him a nice, non-restrictive harness and start with short walks, bathe him bi-weekly and think about changing his diet to a high quality, high protien dry food.
Don't give up, the dude deserves a second chance, with someone that truly loves and cares for him.
Just my two cents.