Sorry your little guy had to start summer off like that. I'm from SoCal, so when we moved up to the Northwest, I was dying to go to the beach. I took my then three year old guy to a beach on lake washington. When we got home I noticed little bumps all over him. The doc said it was "swimmers itch", some kind of algae bloom in the water. I just wait till I can go in the ocean now...lakes are too unpredictable. Of course every time I go to my house in Maui, there's been a shark attack on the news.
Hope you little guy is better soon!
Sabine