All you can really do is push him forward and try to keep him focused.
Keep calm, don't pull back and when he rears kick then stay calm.
My mare has this same habit. To prevent her from doing this, I close my legs on her and push her forward. When she goes forward, I make sure that my outside rein has a firm contact with her mouth and I play the with inside fingers to get her neck to relax. Hope this helps answer your question!