I give my dog a supplement and she's doing really well on it. She had developed a limp off and on for a few weeks so I decided to try Joint Mobility and the very next day her limp was gone. I've continued giving it to her because it's basically the company's Canine Complete supplement, which is like a daily multi-vitamin, plus joint stuff like glucosamine, chondroitin, and MSM. Once we finish this batch I might just switch her to Canine Complete so she gets the daily multi-vitamin.
I have several customers who supplement with one or the other on a regular basis, and salmon oil is another common supplement we sell. I don't know about supplementing to put weight on though. Something like Canine Complete would probably be good to help get him healthy again, but I'd stick with a healthy diet to take care of the weight issue. If he's getting the nutrition he needs his body will get itself in order on its own