Does he have a verbal "slow down" cue? If not, you could teach him one while free lunging.
Are you lunging in a round pen? You can experiment while free lunging with his pressure points. Stepping toward the shoulder can slow them down, but it only works if he's focused on you.
Also, you could spend a lot of time asking him to transition between whoa, walk and trot before he bolts. Maybe even a few changes in direction. Get and keep his focus and he should be easier to control.
Learning never stops