Very diffficult. Our clever grandchildren are too blooming bright by half.
I suppose if you have the time and patience you could try a bit of association training - Plan to go somewhere he likes, and that he knows you are going to, saying firmly as you strap him in "If you undo this we can't go, we will have to come back home"
Then watch him like a hawk, ideally with someone else sitting beside him to see it at once. When he undoes it (obviously it is when, not if, if he is making a habit of it), say immediately "We have to go home now, you can't be in the car with that undone" He will throw a wobbly, probably, but be firm and drive home.
After a very short time suggest the trip again, repeating the warning, and acting immediately if he undoes it again.
Don't get angry, just quietly repeat the exercise again and again until he associates going somewhere nice with being in the seatbelt and not unfastening it
It will take patience and resolve! Good luck.