I had this problem. My TB when she cantered she would bolt forword and not slow down, it wasnt the bit at all in my case. I was to heavy on her mouth what was causing her to go faster..she liked it when i touched then released.

He could be unbalanced and leans heavily on his forehand (and thus your hands) as his natural way of going.

Work on doing lots of transitions - halt/walk, walk/trot, trot/canter, etc. These will help him sit on his bum and push from behind whilst working. When you do canter, only do a few strides then ask for trot, half-halt then ask for canter again. If you practice transitions and half-halts he should start to lighten and use his back end more.

When you canter make sure you have a light contact - don't pull against him as he will pull back and become heavier. When you ask to slow, use your seat and legs - 'hold' him to stop rather than pulling against him. When people say 'forwards to trot' they mean you should slow but keep a little leg on to retain the impulsion so they march into the next pace rather than slumping and slowing.

Do some polework, starting in walk first. These will encourage him to lift his legs and work his bum mucles as well as concentrating on where his feet go - these should also help impulsion from behind and coordination.

Have a few lessons on him so you can learn from an instructor also, lessons are invaluable, no matter what standard of riding you are.