We have a protocol for places like that. We do not eat anything unless that concession stand has no peanuts. We ripe down seats (the backs tend to be the worst because of how folks deal with the shells), take a crib sheet if possible and a plastic bag to put it in. We wipe our should down and put them in a bag at the car before getting in--or wipe them down and have paper mats below that get thrown away. If possible, shoes go through the washing machine at home (this is a great place for crocs).
We don't go to stuff like this often. It's easy not to, because our son hates sports, but as he's grown up this would be done at the circus, a few shows that he saw with PBS characters in arenas, the rodeo and a few college football games.
Even now, when the school band plays at our state hockey playoffs at the big arena downtown, I invoke his 504, and he doesn't go. It's too much to deal with to stay safe (and for something he wouldn't normally watch anyway).
A few weeks ago he was at a grad party where there were peanut shells on the ground and bowls of peanuts everywhere. The grad was from Nigeria--totally made sense. But I had DS use a Clorox wipe on the bottom of his shoes before getting in the car. Peanut protein is such sturdy stuff!
You didn't ask about all this--but we put this stuff into place because of the kind of reactions our DS has had. Your DS' reaction could have been from the peanut protein swirling in the air, but it's very likely it was contact. Peanut protein is everywhere at a baseball stadium. I've mentioned before that my DS has had anaphylaxis from accidental contact ingestion at school. He did get the Epi then--based on symptoms.
I probably wouldn't given then Epi to your kiddo, given what you described.
But it's easy to see how it could have been a reaction.
Fwiw others here go to ball games no problem. Also, there are PF games/sections now.
But I've got to say--our lives have been easier because of DS' interest in dance and theatre, lol. People may be eating peanut M&Ms, but they're not throwing shells on the ground.