Have you pulled the skin back to make sure that there is nothing wrapped around it? This is pretty common, as hair, string or other objects can sometimes wrap themselves around it. Just a thought.
Did he wear a swim diaper? If so, had he ever worn one prior to this? It could be the material used in the swim diaper.
Was he wearing trunks without a diaper? The irritation could simply be from chaffing on the netting of the trunks while swimming.
Just some ideas. If it doesn't clear up within the next 24 hours, I definitely would make an appointment before the weekend. Best wishes!