I have a 3 year old Boxer dog, he has a thin muscular built. What can I give him to put on weight? I've tried all the vitamins in the pet store

I had a female German Shepard once who kept jumping over a 5' chain link fence and meanacing my neighbors who had the nerve to walk on her side of the street. I needed to put weight on her also. I found that feeding her peanut butter sandwiches did the trick in no time (in addition to her regular dog food). She loved them. Pretty soon she was too plump to jump over the fence. Might work for your dog.