Stomach aches can be a sign of something physically wrong beyond the flu or obvious illnesses. So it would likely be worth a trip to your pediatrician to rule out anything more serious.
Once it is ruled out - and if he continues to complain, then it is likely something is wrong with his environment at school. Maybe he's bored, or maybe he's being bullied. Perhaps something has changed with his teacher - scolding, pressure to perform, ... It could be a myriad of things,
My youngest got Yellow Bus Fever a lot (as in the symptoms went away when the bus did, too). His was early signs of anxiety and usually meant he was getting too pressured to master a skill or that something was going on that he didn't know how to resolve. It was rarely boredom. When he was bored, he said he was bored. It was when it was something he didn't know how to resolve that the stomach aches started,
Hope things are better for you both soon!
