Not a a darn thing I can see about that horse that will mean he is bound for lameness. He has a nice hind end. Good hocks, good bone. In fact, his hind end is his best feature.
His shoulder is steep and small, with a low set and ewed neck. However, with correct muscling, you'd hardly notice that. I do not see him as underweight, just under msucled. Making him fat will not make him better looking, IMO.
To sell him , play up his good personality, photo him with kids on top. Make sure you photo from a very square angle to shoe his hind better and don't have him turning and looking at the camera, as this makes his neck bend in even a more ewed way. Have him look forward, and get him curious about something, so he raises his head a bit. You can even have him just barely about ready to step forward and reaching forward for something, like an apple, just out of the photo frame. Someone will want him for his personality, ewe neck and all.