I was swarmed while cutting branches last year. I think I might have even started a thread about it. In any event I got nailed at least 20 times and t'was from head to toe. It / they hurt like hell for a day or so. The swelling and redness lasted a couple of days so what you describe is normal. If he was to get a bad reaction to the sting it should of happened withing the first 1/2 hour or so.
Oral Benadryl will help with the reaction. Several things will help with the irritation if done immediately after removing the stinger if one was left. Wet tobacco helps, or meat tenderizer made into a paste, or ammonia.