Hi S.,
My oldest hated brushing his teeth too and with him it was the tooth brush bonking into his cheeks and gums that he hated, so I got him one of those kids spin brushes. Now all he has to do is hold the brush on each tooth for a little while. He says it tickles. Another thing that helped was when I was still doing it for him, I would stand behind him with him on a foot stool so I had better control, then we would both "make our mean faces in the mirror" and I would hold my mean face in place the whole time and even throw in a silly addition now and again to make it look like I was trying TOO hard to be mean. This cracked my son up and he has had no problem brushing his teeth since because he gets to make his mean face in the mirror. They now have this pre-brush rinse to show kids the plaque on their teeth, I think this may be helpful too. Your husband can have your son rinse with this so your son can see why it's important to brush. Maybe he can tell him that the blue stuff left on his teeth is, oh "special mouth dirt finding miniature secret agents" or something and tell him that his "mouth dirt" will make holes in his teeth if he doesn't brush it away. My oldest was convinced when I told him that the tooth fairy would leave him crackers instead of money if his teeth had holes in them because she uses the teeth to build her castle and she doesn't want any holes in her castle letting cold air in.
I think making it a battle like we did of the bad dirt and germs in his mouth vs. the shining heroes of tooth brush and paste may be a good tactic, especially coming from a dad. If there's one thing little boys go in for big time, it's pretend battle. I would even sometimes make fake "dying" sounds on behalf of the "villians" (which ended up sounding a lot like the Wicked Witch of the West in her death throes) which always made my son giggle.
As you can see, there are many ways to make tooth brushing a fun thing to do. You just have to appeal to the child's sense of humor and his imagination, both of which children have in abundant supply.
I hope this helps. Good luck!