We let DS have toothpaste (the tiniest of tiny smears, unless he gets his hands on a tube...) but he has to rinse and spit (he's getting quite good at that). And I let him choose his own toothbrushes, which seems to work. At least till the novelty of having a wiggles or dora toothbrush wears off, then it's time for a new one.
I don't worry about how long or thoroughly we brush - anything is better than nothing and it's getting gradually better. Don't want to put him off it entirely.