G.,
At this age, I don't think I'd worry too much about brushing your son's teeth. Any teeth that he has now are baby teeth, and he's going to lose them anyway, eventually.
My son is grown now, at 29, but when he was your son's age, I never brushed his teeth. Back then, brushing, or even seeing a dentist wasn't recommended until a child was at least 2 years old, and he never had any cavities, until he was much older.
In my opinion, fighting with him in order to brush his teeth may be so upsetting to him that when the time comes that he does need to brush his teeth on his own, he may be so traumatized by your doing it now, when he hates is so, that he won't want to do it at all!
I would continue to let him watch you and your husband, while you brush, and encourage him to brush his own teeth, along with you, on his own. Even if he doesn't do a very good job, praise him anyway, and try to make brushing his teeth fun, and something that you can do together. By doing this, he won't be traumatized by your forcing the issue, and he will be much more likely to develop good dental habits when he gets older.
Also, you might want to try a few different types of toothpaste, until you find one that he likes. If he finds the taste of the one that you and your husband are using "yucky" (as my son used to say), that may be one reason why he's giving you such a hard time!
Hope these suggestions help!
D.