K.N.
HEy J.: I've been doing the teeth brushing battle sine my daughter was 12 months. UGH! It can't express how much I dread toothbrush time... But it is so important that I am committed to doing it no matter what. Just stick with it. His behavior might be because of a recent bad experience or it might just be another way he has found to avoid it completely.
I suggest you buy/try many, many different toothbrushes. (I could never do the finger brush because my daughter had teeth and knew how to use them OUCH!) Let him choose his toothbrush. Try a cheap mechanical one, maybe Thomas the Train or Spiderman. There;s one with a circular spinning head that worked well for us (until she got sick and then I threw out all the brushes) The rotator head isn't very strong on the grocery store ones and shouldn't hurt his gums. (I suspect the 3+ age warning is because hair could get stuck in the rotator, so make sure you supervise). I also found manual toothbrushes (at Walgreens) that had a suction cup on one end and stuck to mirrors... my child thought that was funny. And always worked to let my daughter try to brush my teeth, then it was my turn to brush hers, then her turn on me, etc.
Oh I hated trying to brush her teeth between 12-20 month! Oh, you've reminded me of the battles... But stick with it. Do it every night no matter what (and baby will realize it is part of the routine) and it will get easier.