This may have absolutely nothing to do with your son's experience, but it's worth a try...
When I was somewhere around 14-16 years old, way back when, I had just finished dinner. As I turned around at the sink I told my mom that I wasn't feeling well. I tried to walk a few steps and, before I could say anything more, I keeled over into a faint on the floor. My mom called 911 as I was coming out of the faint. I said I was fine but she finished the call anyway. Shortly thereafter, the ambulance attendants arrived and did a simple assessment.
Afterwards, we had several tests done to rule out any unforeseen issues, none of which were conclusive. But one of the ambulance attendants suggested that it might have been gas. Talk about embarrassing for a young teen! But I actually think that was what happened. It was probably an astute observation by the ambulance guy.
This is what I felt: sharp pain in my upper stomach, right below my rib cage, enough to affect my breathing. Then everything started to feel far away and then I blacked out. When I woke up, I remember ringing in my ears--a common response to pain. Then I felt fine, albeit surprised and concerned.
It seemed to have been an isolated incident. I don't remember my heart racing, but it might have been. This was over 30 years ago, so I might be forgetting details, but I think I got most of the story right. It was a strange incident to say the least.
Hope this might help.
A little about me:
I am a 47-year-old married mom of an 11-year-old who borders on ADHD.