J., I see that you put an answer to your own question below. That's just not a helpful way to do it! Please take that info and write "edited to add" and put it at the bottom of your original question! That way EVERYONE will see it. The way you're doing it, all new responders will be expected to read every single answer from every person, and you'll be hoping that they realize one is from you!
You dress him in very little, and you can try putting a towel underneath him in the car seat or perhaps one of those "wicking" shirts that athletes wear to wick the sweat away from his body and into a towel below. I caution you on this though, because you cannot have too much material between him and the car seat and straps, or he won't be safe. (There have been a ton of warnings about putting kids in car seats with their winter coats on, for example.)
And have you discussed this with the pediatrician to figure out why he's so hot? I wouldn't use any products without verifying them with the doctor or the Consumer Product Safety Commission.