Have you had him seen by a vet, to rule out mange or fungal infections? If so, and he's blue, which it looks like he is, it could be a condition called Color Dilution Alopecia, which basically means hair loss due to coloring. A dog that has dilute coloring, like blue or fawn (Isabella), has uneven pigment distribution on the hair shaft, making the hair brittle and prone to breakage. Just about all dilute dogs have it to some degree, and some dogs wind up pretty much bald from it. Other than feeding a good quality food and not doing a lot of grooming that could damage his coat, there isn't much you can do if it's CDA.