That must have been very disappointing, but I do still think health needs to be put first. Even more so if they didn't have healthcare. Risking the problem to escalate is just asking for major ER bills, not to mention a dangerous situation for a loved one. (And possible future guilt feeling responsible for "ruining" vacation and causing expensive bills.) There is also of course consideration of the other park employees and guests. It sucks being sick yourself, you don't want to take down as many fellow humans as possible, amirite?
But what if you discovered that a party member was ill before you entered the park, or even a day or two beforehand? Can you reschedule/get a refund for your trip then? I don't know. That's a pretty stressful, sad situation. You should always take care of your health, but perhaps more so before vacation eat more veggies, fruit, drink water and wash your hands more frequently, lol.
I will say though, for feeling your best during the day in the park, take it easy on the junk food, make sure you wash your paws, drink plenty of water and don't ride things that have a high upchuck risk shortly after eating.
Poor kid, hope he feels better. Being sick is never fun, but being sick at Disneyland (or any vacation destination) has gotta be the pits. Sending good wishes that not many others caught the bug while they were there as well.