S.G.
I just flew to Disney World with my 3.5 month old. I wouldn't be freaked, they are easy at that age. Check your car seat, I wouldn't bring on the plane. Just hold him. If he gets cranky, feed him. If you're breastfeeding, just make sure you have a blanket handy. I breastfed at least once every flight. The only tricky one was when I was on an aisle. Lol. Oh well, I will never see those people again....ever :) :)
Room in the changing rooms? Good luck. The last plane I was on they didn't have a changing table. I had to change him ON the toilet...gross. At least I had his changing pad, but I will never do that again. She was stinky, otherwise I would have changed her in my seat, which is what I did for the rest of the flight. Ask your flight attendant when you get on the flight about a changing table.
Bring your stroller, it makes it SO much easier when traveling in the airport. You put it at the airway for check-in, and you get it when you get off the plane (SO easy). Plus, if you need to change him before or after getting on the plane, you can do that in the stroller if you'd like. No searching for a bathroom.
I only brought her diaper bag, a boppy and a blanket on the plane. They aren't very responsive to much at that age, and will most likely sleep. I put some things to keep me busy in the diaper bag, and when I got on the plane I took out a couple diapers and wipes and put them in the pocket in front, and stuffed the diaper bag by my feet. The boppy was a great place for my baby to relax and to feed from. It worked out perfect.
Sidenotes: My baby is hardly ever fussy (so take this advice from that angle). Also, I have no problem changing diapers in public...I do a good job doing it fast and having least amounts of people seeing anything, so that helps w/ the plane experience.
Good luck, don't stress, and have a great flight :)