He is, as you note, underweight. Weight will help him a lot. He is ewe necked and his neck is set low which will make him difficult to collect and get working off his hind quarters. He has an injured right knee. It may be a blemish or not. If he is currently lame, it is not a blemish. If he is sound, and he works and stays sound, it is just a blemish.
His humerus lies a bit flat and his point of shoulder angle is a bit closed. He will not be a good jumper and will likely hang his knees. This may also contribute to him being heavy on his front end.
He has a nice, short back and a pretty nice, smooth coupling. Hind legs appear correct with his hocks and knees set nice and low. He also is NOT butt high.. his ewe neck makes him look that way. Nice withers. Not a bad horse at all.
Give him groceries, a good worming, and have a vet check that bad knee for soundness. Many a horse has a big knee and is perfectly fine for flat riding, trails and pleasure work.