If your son was diagnosed as having a stomach virus, then I imagine that he developed nausea, vomiting, and diarrhea
along with generally feeling crappy. After a few days this usually goes away. It is quite possible that he has caught a stomach bug a few times in one year, especially if he spends any time in day care or pre-school. Kids that age often have not been exposed to a lot of the viruses that can cause these symptoms and thus they get sick every time they are exposed.
On the other hand, your son may have another reason for his symptoms. Two pediatric gastric conditions, volvulus and intussusception, can sometimes mimic viral infections (abdominal pain, nausea, and vomiting). These may only last for a short time before resolving. His doctor
may only explore these possibilities after a viral infection has been ruled out.
I suggest that you schedule an appointment with your son's pediatrician
next time he gets sick. If he continues to develop these illnesses, then he will eventually need additional testing to sort out why he is getting sick. If you get him to the doctor in time, they can do some abdominal imaging to rule out volvulus and intussusception as causes for your son's illness. He may also need immune system testing to be sure that he is able to fight off infections adequately.