Watch from afar, come early to get a good seat if you think he can handle being in the middle of crowds, bring along headphones or earplugs or whatever else you think may benefit. The goal is to have him as relaxed as possible to avoid over-stimulation, so approach that goal with methods that have worked before. I'm HFA myself and crowds for parades have never bothered me because I've always paid attention to the parades themselves, never to the noise from the crowds. Being in a packed crowd and getting yelled at in Jibberinglish is vastly different from being in a packed crowd just to view things. I have nephews and nieces with autism who are more easily over-stimulated, attributed, by me, to their ages, but they are fine in these sort of situations if held by an adult whom they trust.