Using his hind?

Rein backs after transitions and turn on the haunches. Both movements, if done correctly, require a horse to engage his haunches and bring them under him. It will force your horse to become more aware of where his behind actually is and he'll become more responsive during forward movements.

Rode him today, he was stubborn for a little and very hyped up, but I finally jumped him again- the highest we've gone, 2'6! =] On topic, though, his collection was better. I rode him without his S.M. Today and it helped him carry his front end a lot better, which helped his hind too! =]

I agree with both of these posts ^^^ moving the haunches around so he is stepping under himself will get his hind engaged. Remember, that all muscles have to be built over time, so gradually build more of these into your routine over time. Leg yields, turns on the haunches/forehand, shoulders and haunches in will all help teach him that he has a butt! LOL

OTTB's are basically taught to be front wheel drive...so being on the forehand is very common for them. Over time and with correct training, we move their center of balance from thier shoulders towards thier haunches. This is imparative in jumpers and dressage horses as they cannot correctly do thier job if they are weighted down in front. Make sure that you are not adding to the problem.... you need to keep your weight back as well; stay tall, drive them from front to back and use your half halts to balance them.

Will jumping help, or should I get this down before jumping him too much?

I personally wouldn't be doing a lot of jumping until this is improved and the things I mentioned below are at least fairly consistant.

Trot poles, cavaletti's and canter poles and some low gridwork can be used to change things up a bit if things get stale, but the quality of the horses jumping is going to be seriously hindered until he gets off the forehand.