Make sure you keep your heels down! I know it sounds obvious but sometimes this is the answer to problems with feet slipping out of stirrups. Alternatively, you could canter him without stirrups and see what happens.
Or perhaps you aren't sitting straight? Can you have someone stand behind you and let you know?
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare