I dont't know if mine will work on your boy, 'cos it seems that he 'enjoys' the privacy of his mouth, or teeth for that matter. Doesn't want any 'intruders'.
For both my boys, they were willing to brush. It was the rinsing part which was a little tricky, cos' instead of spitting out, they drank the water. Must be really thirsty every morning *laugh*
My trick is a small mini M&Ms chocolate. Not the normal size.
After every brush, they get to eat ONE TINY MINI M&M's chocolate. Nothing more than that, until the next brush.
so you've got to think, what is his favourite yummy. Offer that as a reward.
dont't worry that this will become a habit, cos' eventually that tube of M&M will run out, hence by then, he would be so used to the routine already, then you show him the empty box, and he'll know that its a routine and not a way to get M&Ms anymore...
in fact, before the M&Ms ran out, my boys have stopped seeking for the reward, and went playing right after brushing.