His leg will probably be weak for a while. He'll get tired more quickly, but it won't be dangerous (if it would, they wouldn't take the case off yet).
Lucky he's only 3 because he can probably be lifted and carried quite a bit. That might be especially important for the attractions with mving walkways or the ones where you have to step down in or out of a boat.
Check out the information about Guest Assistance Cards in the disABILITIES FAQs thread. He could get a card or sticker that lets him take a stroller into lines just like a wheelchair. That way, he won't get so tired from walking.