Weight loss for Dogs?

Ok so my dog suffers from thyroid gland problems....and it didn't really flair up until recently. Apparently it's hereditary. Our vet put him on some pills to control his weight but he became really sluggish and depressed so we took him off the pills. I've tried walking him regularly to help him loss weight but he's too heavy and then his legs and paws hurt and soon after h just collapses of exhaustion. is there a food formula or something to help him loss weight..I miss my active puppy we used to have. he's not even old just about 3 years old