E.T.
I have used the CARES harnesses with my girls (they're almost five) since they turned two, and I love them. My husband used to be a lobbyist for the airline industry and did tons of research on the safety of the harnesses - we were both satisfied that they're just as safe as car seats. And they're so easy. Seriously - I feel like the CARES people should hire me as their spokesperson I love the darn things so much. As long as your son is over 22 pounds, the harnesses will keep him totally contained and secure. My girls take naps on planes with the harnesses just fine too...
Have fun on your trip!