Water bottle he's used to will help with the ear popping a bit. Other passengers will either understand travelling with a kid or they won't, group 1 should be fine so long as you do what you can to keep control of the situation, group 2 may get agitated and cluck a bit but you don't have to see them ever again. Stress transmits, relax and do what you can to enjoy your son's first flight with you. Lovely Christmas story really.