I would get one of those baby mirrors for cars, that you can install in front of you that reflects directly on your child. That way you can glance at the mirror just like your car's regular rear-view mirror, without twisting your head around and taking your eyes off the road. Talk to your child and put on some fun music, until he slips his arms out or unbuckles, and then immediately stop the music and the chit-chat. Pull over and like someone else said, don't make eye contact when you re-buckle him. I also think it's a good idea to have a fireman make him promise to stay in his car seat and to check the straps.
Also, make the punishment fit the crime. Is there a place he loves to go, like McDonalds or a playground or something? Make a very simple chart, with a picture of the place (draw some swings, or take a photo of it, or get a McDonalds happy meal bag or whatever). Tell him every time he rides in the car and doesn't touch the car seat, he gets a star. Make the stars go up like a ladder, and when the stars reach the reward photo, he gets to go in the car to that place. If you go out and he escapes, take a star away. You can just cut a simple shape and tape it, or put a Velcro adhesive dot on the back, get as crafty or non-crafty as you like, but keep it very simple. That way, the punishment is logical. In the meantime, while he's earning stars, of course, you'll have to avoid that favorite place or treat, until he earns it. Make sure you sit down to explain the reward system, and the star chart, and talk to him at eye level. He won't understand five-point straps or car seat safety or health risks or moving violation tickets, you'll just have to say this is Mommy (and Daddy's) rule for riding in cars. And make sure that you wear your seat belt. And if he rides with other people (dad, or grandma for example), the rules have got to be followed with consistency by everyone. At first, keep the star chart short (just a couple of stars for a very simple reward). And give him something to do with his hands while he's in the car. Have books, special toys, or something entertaining available. He might just be bored and fiddling with the car seat straps.
Last year, in our town, a mom was driving with her three-year-old in the back seat and he unbuckled the chest clip on his car seat and slipped an arm out. She got distracted and turned to look at him and at that second, the large city bus in front of her stopped. She rear-ended the bus. The car wasn't extensively damaged, she had minor bruising, but the child - due to being partially in and partially out of his straps - twisted around and hit his head. He is now and forever will be a complete quadriplegic from the chin down. He will never speak again because he requires a tube to breathe and swallow. The parents have been quite vocal about it and allowed the news media to document their story to tell others about the dangers of children not staying in their seats and also the dangers of moms turning their heads to look in the back seat to see what's going on. They advise the mirror system.
I'm glad you're being proactive about this and seeking a solution.