Quoting Alyssa ♥s Arion: i have flown about every two months since my son was 6 weeks old. the trip i took at 15 months was the worst out of all of them. are you going to be by yourself?
For me that was the hardest trip because a 15 month old wants to walk around a lot but doesn't quite understand that he can't.
Take lots of new toys, try to plan it during nap time, lots of snacks. Have books to read to him. If you have an ipod or laptop or something that plays videos, try to have some ready for him with some earphones. Coloring books or some sort of magnadoodle or aquadoodle is a good idea. A blankie in case it's chilly. I sometimes let my son stand up in my between my legs just so that he gets the illusion of being mobile but isn't really and we play hand clapping games or something while he's standing there.