For his ears, bring a lollipop for him to suck on during takeoff and maybe a second during landing. My kids really like ringpops. It will stop his ears from popping too much.
Skip the car seat on the plane. He is too big for it. If he's in a car seat, he can't use his tray table and he'll be kicking the seat in front of him too much. Just check the car seat (most airlines don't include it in your baggage allowance) and let him sit in the regular seat. Maybe he'll even lie down and take a nap.
Don't bring a ton of toys with you on the vacation. Just a couple of small things that are his favorites. He won't need much - the in-laws will keep him occupied and he'll find new things in their house to play with. You should be able to do laundry while you're there too, if needed, so don't feel like you have to bring a bunch of extra clothes.
On the plane - bring extra clothes for EVERY family member. I can't tell you how many times I've heard of kids throwing up all over their parents on the plane and they only had clean clothes for the kids. My husband was one of these victims!