Hi S.,
Well, I have two suggestions. Here is the nice way: Take him to the store and let him pick out his own big boy electric toothbrush. Make a really big deal of this. Tell him how proud you are of how big he is getting. All that good stuff and then give him lots of praise when/if he brushes his own teeth. Then tell him you need to check if all the germies (what we say) are gone. That way you get a quick swipe to make sure everything has been cleaned and with the electric head, I feel that more gets brushed with less time. (I am not sure if a dentist would agree)
The second is for the more stubborn (and I have used both methods at one time or another with both of my children). Tell him he is going to have his teeth brushed whether he likes it or not and it can be easy or hard. His choice. If he still refuses, then simply shove (gently, but firmly) the brush in and start brushing anything you can reach. He may cry, but that way you can reach those back teeth. He may clamp down, but that way you get the front teeth and sides. Either way, you let him know you are serious and that his teeth are going to be brushed. I only had to do this once with my girl and twice with my boy. I hated doing it that way, but all other methods failed.
I hope your child is less stubborn than mine, but if not, know they both happily brush their teeth now.
Good luck, A.