Great reply Patagonia...
Might add my twopence worth.
Difficult to say why he's doing this without seeing him. However, it's typically due to horses riding too much on the forehand.
If it is this - I would strongly reccommend not using draw reins, as they allow the horse rely on them for balancing. They are great for building muscle, but only once the horse has relaxed the underneath of his neck and can carry himself from behind.
First, I would trot your horse whilst standing in the stirrups and encouraging him to stretch down - this frees up his back and allows him to step under. If he's not tracking, then give a series of gentle taps with the schooling whip.
Once balanced, step into canter. Don't worry overly about getting the correct shape; instead sit back - don't let him pull you forwards... if your seat is vertical (or even a little too far back), the horse will find it difficult to drop onto his forehand.
Then ride with the leg aids: ensure legs are wrapped around the horses barrel. My instructor says it should work by a near inperceptable squeeze inward at the point all four legs are 'suspended', relaxing when they strike the ground. It should feel like you're 'lifting' the horse.
Again gentle taps with the schooling whip should help them engage their inside hind leg.
This should free up his head, and get him balanced, and a more comfortable ride for you!