Walking, lots of walking. Work on simple things like turning left and right, stopping (with a small amount of pressure), but keep his mind engaged. A lot of ex-racers automatically have that instinct to go-go-go, and that is why going slow is so important. Work on "fixing" his mouth. Use a simple snaffle, and get him responding to it easily before moving on to more advanced exercises.... work in circles and serpentines. This will help his balance. Your goal is to have him walking on a loose rein with his topline extended (so he stretches down over the bit).
If you feel him speeding up, don't lean on the reins to slow him down. You want him to walk slowly with minimal contact, so when he speeds up, change directions quickly (but not aggressively). Make sure you work in an area where there aren't a lot of distractions, if possible.