Ben is not a big fan of toothbrushing. He'll mostly just want to run away. But I've found a safe way to hold him that does the trick. (Of course, he still mostly just wants to suck off the toothpaste, but I'm at least able to get the brush moving around in his mouth.)
We're normally in the bathroom, but one night we were brushing in the kitchen so I asked Lori to take a snap. I have him stand on his stool, or in this case, a chair.
I take his hand and hold it behind my back. I don't have to hold tightly, he likes holding hands. If he pulls away, I can tighten my grip on his hand. If he pulls a lot, or feels like he's going to jump, I can let go quickly and there's no risk to hurting him. But he rarely pulls away, he often will just lean into me, like a hug.
Now if we could just get him to be ok with flossing. But then, even I don't really care for that.