What Causes Stomach Discomfort After Taking Sertraline For Depression?

HI, MY 17 YEAR OLD SON JUST STARTED TAKING SERTRALINE 4 DAYS AGO FOR DEPRESSION. TODAY WAS HIS FIRST DAY BACK AT SCHOOL & HAD GONE TO THE NURSE WITH STOMACHE DISCOMFORT, ALSO APPETITE HAS DECREASED.ALSO SAYS HE FEELS TIRED & DIZZY. HE S ON THE TRACK TEAM SO HE RUNS 6 TIMES A WEEK. I M VERY WORRIED ABOUT THESE SIDE EFFECTS