Poor baby! I don't have any knowledge about this really. The only thing I can say, which I'm sure you already do, is be sure not to dismiss or laugh off his fear. Even a trivial nightmare about green monsters (I know, not a good comparison to his very realistic fear) can be traumatic to a child. He needs to know you are listening and you care. I would say lots of prayer followed by distraction is probably what we go for. I don't know about heart conditions, but I know anxiety can cause increased heart rate also, so if it's possible to help him be calm, that might play a small factor in keeping the heart rate down. Good luck to both of you and I hope he feels better!!
"I'm so crafty, I make people" DD (01/06) and her much anticipated sibling (07/10) with my sweet DH (07/02)