It sounds like a classic case of post-shave alopecia. Its a typical risk for any fur-bearing breed that gets shaved. Either the hair just doesn't grow back or it grows back in patches. Mostly its harmless but it can be a sign of futher complications like black skin disease, which can be hormonal or auto-immune. Either way if his blood panels look good for thyroid, hormones, and kidneys it shouldn't harm him (these are probably NOT the blood panels he had for teeth and neutering). I've seen cases of black skin disease where dogs get huge open sores though so make sure they don't shave him ever again if it grows back. Cutting a fur-bearing breed with scissors is better. I don't know what starts it but shaving seems to irritate the skin enough on a dog predisposed to alopecia and then it goes from there. It can happen anytime in a dog's life too, even if previous shavings have been fine. I can understand he had to be shaved due to his condition, but medical is the only reason a fur-bearing breed should be shaved. I fhe was shaved when they got him and had a recovering scar from the collar I bet somebody was treating him when he got away.