He definitely needs a hearty breakfast with lots of protein before taking the medicine, if that isn’t happening already. He also needs to be drinking lots of water.
He could be using this concern to get sent home from school too. At that age (3rd grade), my son pretended that he was gagging over the garbage can in class every morning, trying to get sent home (he had actually thrown up and been sent home, then continued to use that since it worked so well). Talk with him and make sure there isn’t something causing him anxiety and stress at school (which could also be making his stomach hurt, btw).
ADDitude Community Moderator, Parenting ADHD Trainer & Author, Mom to teen w/ ADHD, LDs, and autism