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?

Your child still will be in risk of contacting the hfmd if he touch the places where kids with hfmd touched before. Just to make sure your child don't put his hands into mouth or rub eyes or nose and sanitise your child's hands as frequent as possible in the park.