Grief, hope your ds wasn't aware of the arguing and aggression. Very bad form for it to happen in the school.
How long have you been with DP?
Generally your ex can't control who attends events - especially sports day when it's an open event and no one needs to sit next to each other. I think this is different to parents evening where the parties with PR should attend - it is definitely more awkward if you have 3 or 4 adults sitting opposite a teacher!
Your dp is wrong to have argued and now sulk. Hopefully this isn't a pattern of behaviour but watch for this as its a red flag.
All of the adults should have focused on what is in the your sons interests, not if your dp "deserved" to be there, that's his internal need to be rewarded. Being a step parent is very tough...it is often work with less reward but that's the price a step parent pays and not everyone is truly up for the job.
Equally your ex can't dictate who is around ds..would he object to a grandmother being there?? And your exh shouldn't be allowed to get away with aggression.
Given you have at least 11 years of parental events you should agree with your ex what happens. If he has a longterm partner is he likely to want to include them?
If you can't get an agreement then consider mediation with your ex...would definitely be worth the investment if it reduces conflict for your son.
You son is actually the one in the middle and all he wants is to enjoy sports day without conflict. The adults in his life let him down because they chose their feelings over his.