It may be real, but anxiety related like Disney Doll suggested. When I was in second grade, I went through a spell where I was afraid to eat lunch in the cafeteria. Don't assume that he is trying to get out of school. It may be happening because he's worried about something.
It was early in the school year and one day we went to lunch. After I got my tray and sat down, I realized that I didn't feel well and couldn't eat. An older teacher was on duty and insisted that I wasn't leaving the cafeteria unless I ate. So I forced a few bites down and . . . you can use your imagination. I seem to remember it being something like a scene from The Exorcist. Maybe that teacher though twice about forcing a kid to eat after that.
So I went home and recovered from the flu. When I went back and my teacher asked who was eating in the cafeteria, my tummy started to churn. Just the thought of going in there and the smell of the cleaning products that they used made me so anxious and upset. Soon I was getting ill in the morning just thinking about going to school, because I knew they would expect me to go in there. I think I was also afraid of dealing with that teacher again.
You know what finally worked? Mom explained the situation to my doctor. He gave me some "medicine" that was a placebo. He said I'd need to take it twice a day for a week and then once a day for another week. At the end of that, I would NOT have tummy aches anymore. The medicine would fix it. He sounded so sure that I believed him. At the same time, my parents assured me that I could walk home for lunch every day. I would never have to eat in the cafeteria again if I didn't want to. I was so relieved and never had another problem with my morning sickness.
Kids have a way of blowing things out of proportion. Does he like his new teacher? Are his friends in his class? Is he having any problems? Any bullies or anything like that? Is any subject hard this year? Are they asking him to do something that he's afraid will make him look foolish? (I ask that one because I once had a teacher that wanted each of us recite the multiplication tables in front of the class and I didn't know mine. I was terrified that my friends would think I was dumb!)
I'd have a heart to heart about how school is going. Don't ask him outright if anything is making him "sick". He probably couldn't tell you if that's the case. I didn't understand what was going on with me at the time. Just try to find out if anything is bothering him and if he likes school this year. It might give you some insight.