I agree you left it a little too late so now he is a bit scared. A few things you can do though:
1. Buy a flashing toothbrush (waitrose, boots): they come in colours (let him choose) and flash for 60secs (length of brushing);
2. Let him brush his teeth in the morning but do brush yourself in the evening;
3. While he brushes make him sing smthg like arpeggios. It sounds ridiculous but it does work. It distracts their mind, it makes it fun and then you can call it the brushing song.
4. Brush your teeth at the same time and sing the song too.
If all fails you can call a dentist and make him complicit with so you all have a friendly fun chat, there is a peppa pig one at the dentist and finally get his nursery involved. In my youngest one (on the request of one parent), one day all the kids were to bring their toothbrush abd toothpaste. They were shown how to brush, why brushing, etc. I ve been told i was doing it the "wrong way"
when she came back!
Good luck.