When one of my kids is really afraid of something, I back off for a few weeks - no mention of it, no alluding to it. Then I am really enthusiastic of my own doing of that thing - have husband and older kids in on it - a big treat, something big kids do, etc. But ignore him when you are doing it completely.
Make sure he is not eating any candy (or those fake fruit snacks) or soda - and only 10:1 water:juice. If this is a change, just nonchalantly but clearly say, "It's okay we don't brush your teeth. But I can't give you x when we don't brush them. It's your choice." Be cheerful and move on to the next item of the day. If he persists, nonchalantly say, "You can have x. I need to brush your teeth first though. It's your choice." (Make sure you get to brush them first - he has a reward he is choosing - and do a real cursory, short, successful no way he could ever gag brush.)
I hope this helps.