IMO, the ear pressure thing is overblown. What I found was that it really didn't bother ds any more than it bothered me. The only thing to really be concerned about is if there's an ear infection, then that would be truly painful when flying. Check with your ped asap, if you have any concerns. DS and I both contracted
bad head colds on one trip and the ear pain was excruciating on that flight, but on every other flight, it hasn't been a problem. And usually, IF there's a problem, it's on descent when you will notice it the most. Descent starts probably about 45mins prior to landing. This is when you want to give your LO something to suck on. Swallowing is what clears the uncomfortable pressure in the ears. Hopefully, you will have a very smooth flight and your LO will sleep most of the way.
I also recommend bringing and using a car seat on board. Not just for the safety factor but because it will make all parties more comfortable.

If you are bringing the car seat anyway, it's also a lot less likely to be damaged in transit if you take it on board with you rather than check it. On that note, if you do check it, take it to the gate and check it there. Much less likely to be damaged or lost this way. If you're using DME, and not renting a car, you technically won't need a car seat on the ground. We always brought and used one on the plane anyway and I always felt it was worth the extra effort. I know plenty of people don't agree w/me but jmo. also, just fyi, belly belts and the like (snugglies, carriers, slings/wraps, and those baby b'aire devices) are not allowed by the FAA during take off, landing, or taxi.
Some airlines won't allow their use at all during the flight, just so you're aware.
My only other advice is to bring a clean shirt for the parents, and a change of clothing for the LO, just in case there's a "blow out" or LO gets sick. I won't bore you with the details, but I didn't pack a change of clothes once and it was a big mistake. We usually have
two flights, one leg being 3 to 4 hrs, so your
one 3hr flight really shouldn't be all that bad! Truly
Hope you have a smooth trip and a magical time at Disney!