Sea bands, make sure he wears them on both wrists at the same time, one is not effective. Also teach him breathing techniques. Teach him to focus on a point near him that is non moving and hopefully he can't see out a window, such as one of his shoes, a sticker you place on the seat in front of him, etc (this is probably against some recommendations but I find it works for me and my ds). Some people can focus on a non moving object in the distance, like the horizon. He may be too young to understand that concept but worth a try if you think he can. Closing his eyes may work too - doesn't work for me. Not sure if he has any triggers but sucking on a mint or ginger something similar can be helpful.
My DS is 7yo and also suffers from carsickness, although he's getting a bit better. There are so many different triggers for my DS: stomach too full, stomach too empty, driving through lots of light/shadow--winter bare trees with the sun shining through them--acts like a strobe light, if it's too warm, if there are too many curves in the road, if we hit too many stoplights or make too many turns, etc.
What has helped my son: driving with the windows down, even if they're only cracked an inch or two (yes, even in the dead of winter)--fresh air is the biggest help. Plus, the sensation of the air hitting his face helps his brain/body process that he really is in motion. We just returned from a trip to Walt Disney World and he didn't get motionsickness once! (Of course, I avoided rides like the teacups and carousels, but still, I was so amazed and relieved.) We also have a solar shade on the rear passenger window next to his carseat to help block the worst of the sun. And he wears a wide-brimmed sun hat to further block the sun, or if we're driving at night, to help block oncoming traffic headlights. I ride in the backseat with DS and I keep my hand on the back of the crown of his head, applying gentle pressure. You can try having your son put his hand on the top of his head there and gently press down, once a second for ten seconds to see if that's enough to help re-set his system. Oh, and we had DS's vision tested and got him glasses. Once he finally started wearing them, it made a noticeable difference.
Unfortunately, the sea bands didn't help my DS, but we found an electronic wristband (which is no longer made/sold) which does something similar--stimulating the nerve to regulate stomach contractions.
He also knows to focus on the horizon. Sometimes munching on pretzels or sipping ginger ale would help him; sometimes it ended in puking anyway. My DS also has sensory integration issues, so he won't even try crystalized ginger (I love the chunks from Trader Joe's--they calm my stomach right down, but it did take me a few tries to get used to the incredibly strong flavor).
My mom's an OT, so she advised us about the hand-on-the-head thing and she also showed us how to do the pressure/brushing technique (and gave us the brush), which we only do once a day, right before bed, b/c any other time of day DS won't tolerate it. There's also these special CDs of altered music (certain tones are distorted/over emphasized) that's part of listening/sound/music therapy that DS is supposed to listen to for half an hour a day, but doesn't. I'm sure if he'd cooperate it would also help. So basically, find an OT with specialization in SIPT (sensory integration) to find out more about some of the above.
One last thing--for really long road trips (our family lives 5-8 hours away), we used to give DS children's Dramamine; however, he hated the orange flavoring to the point where trying to take one made him puke. It did zombify him, though. Then we switched to children's Benadryl (which has the added bonus of helping the ear canals drain) for a couple years (though it also zombified DS), but he's again reached the point of hating the artificial flavoring (cherry and grape are available) so much that he'd rather suffer from the motionsickness.
I don't have any other good tips other than those already offered up. I just wanted to sympathize. I suffered horribly from motion sickness as a child, well into early adulthood. Sea bands did not help.
Dd suffered a little with the same thing when she was younger. I always insisted that we stop the car whenever she told us the symptoms were getting bad but before she was in real distress. We would let her get out, walk for a bit and regain some sense of well-being. She rarely got to the point of actually vomiting. When I was a kid, I hated being forced to tolerate it until I was actually vomiting before my parents would stop the car. Now, I understand why my parents did that because my symptoms were terrible and persisted for a long time and our car trips would have taken twice or three times as long as they did. However, I wonder if the reason I suffered for so many years whereas DD hasn't is partly because we didn't let her symptoms become extreme while we were driving. For me, there became an automatic association between riding in the car and suffering motion sickness and I would often feel it as soon as we left the driveway. I think some of that was a conditioned response.
A few people have mentioned using food or mint or ginger. One other warning I have is to avoid using the same flavour of gum or hard candy or other food. I was always given the same flavour of spearmint gum to chew as I got into the car. I obediently chewed it for years but it didn't help at all. I would get sick anyway. I now cannot tolerate that taste or smell because it is forever associated with motion sickness. Oddly enough, I'm okay with other mint flavours like peppermint and wintergreen.
I get car sick, and for some reason gatorade helps settle my stomach. I think it is the electrolytes, so maybe pedialyte would be better to try with a child to avoid all that sugar. I do have to drink it pretty much constantly though, so I wind up having to stop to pee a lot if we are driving a long way.