D.W.
You can look at it a few different ways.
1. You're creating good habits moving forward that will likely continue the rest of his life
2. In many cases the health of your teeth is genetic as well as based upon your detail to cleaning
3. I'd visit a pediatric dentist to see what the health of his teeth is and let that be the guide for how frequently you're engaging in brushing.
4. Be careful not to impose your own stresses about it onto your child - he may begin to resent it and move to wanting to do it less and less
Our kids like brushing their teeth and want to do it twice/day. They both go to the dentist often and have had no issues (3.5 years and 22 months) other than some injuries that we're monitoring.
A lot of it, too, has to do with how healthy their diet is as some foods will certainly cause more stress on the teeth or linger between brushings more than others. I'm sure you and your fiance can find a compromise that you're both happy with that will continue things in a positive direction for your son.