Do you feed the dog low fat food? We have a 10 year old Lab/Shepard mix and he weighed in at 106 when we got him from a lady in New York and he could hardly walk. We put him on reduced fat and reduced amount and he dropped weight like a stone and he is now even running around the backyard and while he still needs to lose weight, he is much healthier now. But, I agree that you need to take him to the vet and have him tested for thyroid and anything else that might be causing a problem. Good luck and keep us informed.