Ignore the total wank about 'this is why kids have no resilience' ... from the people who probably came first in every event from the age of 4 onwards...
Unlike every other activity in school, sports day humiliates those who suck at running (And its always fucking running its never anything else!) in front of the whole school, staff and often parents too.
At 43, I can still remember the frustration and humiliation of coming last every. single. fucking. time. Knowing nothing I did would change that. I had short little legs, and unknown to me, a disability that was making it ten times harder for me to run than the next kid.
My Mum had an in-built hatred for competitive sports for kids anyway (she took part in pretty high level Fencing competitions herself so she wasnt totally anti-sports!), so she took to sending in notes saying I couldn't take part due to sprained ankle or similar things.
Did me zero harm, plenty of resilience here, have survived some pretty horrible stuff in my life and I'm doing alright!
@MissHoollie Take him off for the day or send him with a note saying he's still healing from something and can't compete - whatevers necessary, but don't subject him to the evils of sports day if its causing him this much distress!