Is the dog getting enough excersize to keep his weight down?
Has he been tested for Hypothyroid? <- my dog has it & she can gain a crazy amount of weight. When we got her from the shelter she was 170lbs. Thats 70-90lbs over weight! Shes lost a lot the 3 months we've had her & she doing great.

If thats not it, atleast get him onto an all natural diet, so hes not at rist of being poisoned!