You need to take him back
to the doc. The dosages/meds can be changed
to prevent the racing
heart. It sounds like you did not consult the doc about stopping the med, which in hindsight probably was a mistake. It is not related to minecraft.
Actually, if his breathing gets any worse, I would get in to see your doc as soon as possible. Tell them when you make the appointment what is going on. And if, he starts to spike a temp, get him immediately to the doctor or emergency room.