A.S.
We just went to Hawaii a few months ago with my 3 year old and five week old baby!
Sun, sun, sun. Just get them out in the sun as much as you can to adjust to the time. I don't recall really having any sleep issues when we traveled there. Bring a bunch of small news toys to introduce to him every hour on the plane. We brought about 4-5 things when I traveled with my oldest son when he was small (he has traveled extensively for a 3 year old: Japan, Russia, L.A., Ohio. Russia was when he was 2 and the plane ride was a grueling 12 hours and the time difference was 10 hours!!!). We brought new hot wheel cars (I think 5 in all) and when he got tired of the first one, we gave him a second one. It worked great. I always bring a DVD player, too. That occupies about 2 hours of the 5 hour flight.
Just go with an open mind and a good attitude and I think you will find that kids adapt easily. have a great time!