My biggest advice would be to make sure YOU don't get heavy on the bit. If you get all caught up in trying to slow him down or getting him to stop, you'll be using heavy hands and doing a of of pulling. Which in turn will make him grab the bit and push forward with it, leading to heavy forehand issues.
If you find yourself in this position, I actually suggest moving down to a super light bit (for now) such as a fat loose ring hollow mouth snaffle. From here, work on maintaining a light, consistent feel on your reins, but enough contact that you can feel your horse on the other side. A lot of dressage work will go along way, helping him get supple and sit back on his haunches (picking up the forehand).
It's a lot of work but if you stick with you're sure to see results! Good luck! :)
If s/he likes to lean on the bit, try a Waterford. Half halts also work wonders
Have you tried lunging your horse before work so he can relax before riding?
I would advise you to do a lot of simple exercises to allow him to concentrate first. Then when he tries to grab the bit a little too heavily, do a gentle half halt and straight after push him forward to keep the impulsion.
Ride a lot with your back and legs and less with your hands to prevent him from becoming too heavy.
Don't forget that if he is a former racehorse, he as taught to do so all his life and it will take time for him to adapt to his new life.
Good luck, hope it helps :)