Disney is one of my favorite places in the world.... you are sooo lucky! My kids were little babies when we started going. You'll love it. Its excellent that he has his own seat on the plane. If the carseat doesn't fit in the seat, they will just store it on the plane. Just do your best to bring stuff that will keep him busy and occupied. Make sure he has something to drink or chew on while ascending to keep his ears from hurting. Don't bring toys that make lots of noises because people will tolerate a busy child longer than they will tolerate hearing a cow mooing or a bell dinging for a whole flight.
Make sure you allow plenty of time through the airport. 2 1/2 - 3 hours, especially if you are travelling during peak season. Are you travelling over Spring Break??? Busy time if you are... just do your best to take advantage of any early or late hours that will be offered if you are staying on property.
Make sure you follow the FAA guildlines on liquids to a T. That means... if you have only 2 ounces of liquid (i.e. sun-screen) in a 6 ounce bottle, it will be confiscated if it is in a carry-on case - they won't care... the size of the bottle can matter too, depending on the person in charge. It's best just to check luggage with all liquids.
If you need any specific Disney question answered, just personal message me.
Have a Magical Time!