C.P.
I went through this with both of my boys. They saw it as a symbol of defiance to press my buttons and get out of their seats.
First, if you put on his seatbelt and he immediately unbuckles, don't turn on the car or drive until he leaves it on. Tell him "The car can't move unless your seatbelt is on. We have to ALL be belted in." If he unbuckles while you are driving, pull the car over and refuse, again to move unless he is buckled in. Flipping the button part of the buckle over also makes it harder for little fingers to press.
IF that should fail....Well, I think it truly depends upon how you parent, but I beleive that certain things do merit a swat on the butt. Dangerous things that must be stopped. This is one of them. With my oldest, he unbuckled and I pulled over, got out of the car and spanked his bottom. This was, of course, after I had already explained to him that if the car is moving the seatbelt MUST be on.
My second child was stubborn. It required my giving him a little scare to help him understand the importance of wearing a seatbelt. He was 3 or 4. A light tap on the brakes on an untrafficked road is enough to toss him forward a little. Not enough to hurt, just enough to give a little burst of adrenaline because he's unrestrained. My son cried a little and was HAPPY for me to buckle him in. Don't do this if your child sits in a middle seat where they might hit a center armrest, and certainly do not try to send your child plowing into a seat. You're not trying to give whiplash, just make it clear that the car seat is there to keep him safe and he isn't safe if he isn't buckled in. Yes, I know it's a bit unconventional, but it worked like a charm.
Sometimes we moms have to be inventive to send our children a message about real life.
To this day both of my children buckle up without being asked and will not allow anyone to ride with up without being buckled in properly.
Good luck!