I'm told hfmd is not airborne, so is it safe to take my son to outdoor places like parks if I ensure he doesn't come in contact with other kids?

Even if he doesn't come in contact with other children, he can still touch surfaces that might have traces of the virus. Perhaps you can keep your child clear of places that could be hot beds for HFMD like playgrounds and parks. If it can't be avoided, just make sure your child doesn't rub his eyes or put anything in his mouth that he shouldn't. Sanitize his hands and get him to take a shower as soon as you get home.