A week or so ago someone had a thread about teeth brushing, I didn't answer then but have, in the last week made a HUGE breakthrough with Littlemad and teeth brushing so I hope this helps.
He would faff about and generally be a nightmare, it was non-negotiatble but wasn't very effective and I suspect we'll reap the rewards of that in the near future. But I wasn't prepared to physically pin him down.
However, I've just managed a major breakthrough and it now doesn't take 15 minutes to get ihm to pen his mouth.
I tell him that if he keeps his mouth open and still I can be more gentle when he screams the toothbrush hurts him. (It doesn't hurt him, he just knows full well if he tells me something 'hurts' I'll pay attnetion)
I ask him which teeth I should clean next - top left back etc. This gives him some control and involvement.
That combined with a blue star sticker on his t-shirt seems to have done the trick for him.
HTH others.