Why is my yorkie not gaining weight anymore?

I bought a yorkie prob 1 1/2 yrs ago and have had a lot of health issues but he usually pulls through. The breeder we bought him from was on the news for inbreeding. My dog has always been very quiet and scared. He was on antibiotics recently but doesn't seem the same anymore. He has always been a very healthy dog but lately he eats and just does the restroom right after every meal. I have noticed he is looking very skinny and his left leg seems fragile. I don't know if the leg has anything to do with it but if anyone has a clue what could be wrong please let me know.