There are products available to prevent kids from unclipping (often used when a child is much older but developmentally delayed). Look around for them.
I'd also, as safety permits, pull over and do time outs right there. My friend realized his son unbuckled on the highway and he said he's not entirely proud of his reaction, but he basically flipped out, found a safe shoulder and gave the kid a bit of a scare because they were doing 65+ and his son was bouncing around the car.
Also, what do you offer him in the car? DD has a backpack and I fill it with snacks, a drink and some small toys she probably forgot about. When she gets tired of sitting, I toss that to her. I also have kid friendly music for her so we can sing along.