I do think that muscle will make him look 100% better. He is rather thin for a 5yr old Appy.
You cannot fix that fact that he is downhill. And really it does not affect his performance. Some of the worlds best show horses are built downhill. The only problem I find with a horse being downhill is that they are little rougher to ride, my riding mare is a bit downhill!
You can't fix that fact that his neck doesn't tie in right but building up some muscle mass will definitely disguise that fact.