Not weird to get a stomach bug in July at all. When I was little, I would get a stomach bug EVERY TIME I had a break from school over spring and summer. Why I was so susceptible to them, I don't know. Where I got them from, I also don't know; I was never around other kids, just my parents and grandparents, and they never got sick.
Well to be fair, it wasn't always a stomach bug. Sometimes it was a normal flu.
Hopefully he'll get it out of his system quickly and no one else will catch it. Getting sick on vacation is the worst!