howrsegirl, if you are able to, you may want to consider subscribing to The Horse Training Channel. They do have some bits of their clinics posted on youtube and there are quite a few exercises that could help you with this horse.
Self-carriage comes from the back, and it's really hard to tell what is going on with this horse without seeing video of the issue. I would throw a guess out there that he's getting heavy on the forehand and doesn't know how to hold himself and carry himself, or doesn't have the strength to do it, so he is basically running off a bit because of that. Hard to know without video though.