Persistance, and praise.
If you can get him to say "Aahhh" or roar like a lion that's a good start. Maybe practice with him at numerous times of the day so he learns. Quickly and gently brush over his teeth while his mouth's open and happily say "Aahhh" or roar like a lion while doing it. If he focuses on you playing with him he might not even notice you brushing his teeth.
Or singing "This is the way we brush our teeth" might keep him entertained.
It will take time for him to co-operate, but the important thing is keeping it a positive experience for him.
Another suggestion is let him chew on his brush or wet facewasher so he can get used to them then maybe sneak in a quick wipe over his teeth.
