Get a trainer. I had the exact same problem as you're having, except my filly was 2 at the time. Raised her since birth, etc.... Wasn't being firm enough, turned my filly into a bit of a terror. Waaaay too pushy... So I got my coach to give me some groundwork lessons, and she did some training sessions with her as well.
Worked wonders, SO glad I did it.
Good luck with him!!