Hubby's family has awful teeth. Hubby's baby teeth crumbled a bit, as did FIL's. FIL was in full dentures by 50. DS started teething at 4 months...the bicuspids on the top came in yellow, and as they grew bigger they turned brown, then they started to disintegrate. Despite everything we did. We were searching for his first dentist before he was 1. Good dentists are HARD to find.
Anyway, on top of all this, as we tried to keep those teeth OK until we could find a decent dentist and tried to keep it from happening with the others, he *had to* have his teeth brushed. And he HATED it. And he's always been long and strong, and can easily physically overwhelm me and even hurt me.
We are very Gentle Discipline oriented, but not with toothbrushing. We did what we had to do. One toothbrush between teeth on the left side while we brushed the right. And switch. That would work for awhile. Then we had to switch it up. etc. It wasn't pretty, it wasn't fun. It was awful, especially given how gentle we try to be with him in all other aspects of his life.
Good luck.