It sounds like your son is dealing with some fairly classic anxiety symptoms. He had a very frightening health experience and now he is focused on his health. (We call that "health anxiety") Since he has seen his doc and gotten a clean bill of health, then it's time to look into some therapy to get at the root of his concerns. Another aspect of having health anxiety is that we tend to not believe the doctors when they tell us we are OK. Some of us have gone to several, getting the same results and still, we refuse to believe there is nothing wrong with us. It's a nasty condition which can get very expensive as we go from doc to doc, paying over and over for the visits and the tests.......money far better spent with a good therapist.
I hope you can convince your son to seek out the help he needs. Living with health anxiety is no life at all.
Peace
Greenlydia