Ooooooh, I feel your pain. All four of my kids have gone through various phases of that - it's just like nails on a chalkboard, isn't it?!?! Don't let it freak you out too much ("yeah, thanks, that's a BIG help, I'll treasure THAT advice always!"). It IS perfectly normal - he's old enough to know that he doesn't like it, but not quite old enough to vocalize WHY, to understand that this is how we get someplace interesting, or that sorry, kiddo, but Mommy is, too, the boss of you. Deal! ;-)
There are two things I find particularly helpful: distracting them with "silly" or getting them absorbed in the process ("Where does THIS buckle go? Does it go on your toes? Does it go on your nose? Does it go around your elbow? Can YOU do it?").
The other is the time-honored BRIBE. One particular favorite for all four of mine was a battered up purse I got at Goodwill. It had a billion pockets and zippers and buttons to play with - good times! I'd tuck other toys and surprises into it, occasionally even snacks!! Oh joy!! Oh rapture!!! I'd put it on the floor next to the carseat and then say casually, "OK! Now, you get in your seat and then you can have the bag!"
Once they realized how this deal worked, they tended to swarm right on up into the seat and let me buckle them up - you never know! There might be something with lights that flash! There might be something that goes 'beep'! There might even be (dare I hope for it?) a dum-dum sucker!! (Hey, it could happen! Or fruit treats. Fruit treats are good, too.)
Still had the occasional tantrum, though. At those times, really...take a deep breath. You're a good mom. He's a good kid. This phase will pass. It seems like *forever* right now, but it really will be gone before you know it.