I have flown with Layla- the flight was about 3 hours.
Take a look at this website-
Dog Jaunt : Traveling with a small dog. They have great info on flying.
I would NOT do a sedative. you don't want to be up in the air and having him have a reaction. That would be awful.
The best thing you can do is give him a good, long walk before you go. Layla slept the whole time. I kept giving her treats here and there to let her know she was ok.
These are the BEST bags.
SturdiBag? large by Sturdi Products | Sturdi Products
I ordered 3 other bags before settling on this (didn't want to spend the money- but you kow the saying- you get what you pay for!) bag. It is wonderful. The top is not hard so it can squish under the seats.
Do you best on your lay overs to walk him and get him to eat. You might want to line the bottom of the bag with a pee pad and bring extras with you- that way if needs to go you can bring him to the bathroom on the plane or in the airport and let him go on the pad. be sure to have great treats with you to get him back in the bag when you need to.
Try to bring a blanket if you can to wrap around his bag- sometimes the floors of airplanes are VERY cold.
I hope he gets to go! I'm sure he will surprise you with just how good he is on the plane! Be sure to get him used to his bad before you go.