L.W.
The best recommendation would be to get him an electric toothbrush and let him do it on his own. When my kids were about that age I tried many different things since they can have tender gums and be "resistant" at times. Sometimes you may just have to put some toothpaste on a wash cloth and get the teeth clean that way. As my daughter got older (she is almost 3 now), I would convince her that her baby dolls were telling her to brush her teeth. She listens to the baby dolls better than me sometimes. Sounds silly and goofy, but whatever works! I have had to restrain both my kids if nothing else was working. They get over it!