This may be a long response but really feeling for you so maybe my experience can shed some light? I was your DS age when I had psychosomatic stomach pain whenever upset. I had been sick and vomited in a public, very embarrassing way (on stage performing in a recital) and it was very traumatic for me (though no one seemed to realize that at the time) . After that, whenever upset, I thought my stomach hurt. A lot.
This caused me to almost be held back a year in school due to attendance, I actually missed or was sent home over 1/3 of the year's school days. I quit all of my extracurricular activities despite my parents protests, I simply refused because of the stomach pain.
For a year I would do nothing. My parents were not at all understanding, a point that sticks out in my memory as often they did cause my anxiety. Having been told by doctors that there was nothing physically wrong with me, they would get quite angry whenever I said my stomach hurt, which was often. Eventually a sympathetic doctor (who I now think may have been a child psychologist?) talked to me about my fears, the original trauma, and other things that I could do when I felt upset to help me cope.
I needed some thing to distract me when I was upset. With the help of that doctor, I did find other things I could do when I was upset that helped to distract me, which was how I managed all through school. They probably weren't ideal things, but they definitely helped.
TBH though, even today (30ish years later) if I am very upset about something that I have absolutely no control over, I have to catch myself when my brain says "my stomach hurts" - and recognize what it is.
Don't know how much help this is to you but I would suggest trying to show him other, physical things he can do when he is feeling upset.