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?

Like what the rest have mentioned, HFMD is highly contagious and could spread through contacting the body fluid of the affected person. I can understand how you would like to take your child out for some fresh air especially when he is probably very uncomfortable with this. However, you can probably opt for a short walk in the park instead and not having on the playground to minimize the risk of spreading to others. Hope your boy gets well soon! And take care of yourself as well because adults could also get infected.