My dog refuses to go for a walk!?

Dogs have no way of telling you if something is wrong with them. I think you should take him to the vet get things checked out. Your dog might be having some joint problems and he's sore. And since it sounds like the dog has been basically unexercised for a long time your just finding this problem out now. If hip or joint problems are the problem the vert will recommend some foods that will really change the dog to be more active.