I was wondering what kind of virus can cause fever, stomachache, headache and should I be overly concerned?

Good evening, my son, soon to be 11 years old (in January), started to have a stomach ache and a headache on Saturday night (2 days ago) for no apparent reason. I gave him motrin (12.5 ml) in syrup. The next morning he told me the same thing and his throat was irritated so I gave him the motrin again and he put a magic bag on his stomach. He still had a good general state (he played and ate normally). This morning, same thing...as he was coughing and sniffling, I thought it was a little virus but I did a covid test which was negative. He ate lunch normally and I gave him some tylenol. When he came back from school, he was tired, cold and quiet...I took his axillary temperature (37.8) so I gave him tylenol again. He didn't have much supper but ate a lot of snacks. A friend of mine's two sons had this a week ago and their fever lasted 5 days then stopped so she didn't go to the doctor. My son is epileptic (relapsed (was 2 years without medication and without seizure) it came back with absences) but controlled by medication (epival) and also takes concerta 27 mg in the morning. He weighs 57 lbs.