Nausea could be caused by nervousness. I remember when I was in 2nd grade for a period of time, my stomach was always queezy. The teacher was very mean and old. She would yell a lot and hit your hand with a ruler (this was back in the 80s in the south). It sounds to me like something might be going on at school that makes him upset. Have you asked his teacher and waht does she say? Is there a bullying situation? Lots of things to ask and wonder about. Of couse, it's hard to know for sure what goes on in the school as we only get LIMITED info as parents. Is he nauseated when he's off school?
It sounds like he has a social phobia which needs to be addressed with a specilaist. He is actually sick from the fear of going to school. Get him evaluated and go from there, you can nip this in the bud now. If he is nauseated all the time, get him checked out by his pediatrician and if all is fine, you need to address his anxiety. Good luck and take care.