Maybe just try giving him the toothbrush? I did this with mine and let him stick it in his mouth and bite on it and get used to it. You may have a few nights where the brushing doesn't actually happen but better than making an issue out of it. My DS enjoyed having it himself and got into the swing of the brushing himself with a little hand from me.
Also, try doing it together? I sat on the floor and we 'brushed' together and he copied me.