At close to 2.5, even if your plane does have a changing table, you won't be able to use it with a child that old. They really are meant/made for smaller babies, not toddlers, even young ones.
My kids at that age had fairly regular bm cycles. You may be able miss a bm entirely if you're not flying during those times.
Here's a suggestion from a mom who has flown with 2 in diapers at the same time. Put DS in a pull up for the flight. If he is in potty training mode, you may get lucky and the lure of a new potty experience may encourage him to go in the big potty. If not, and he has a bm, find the biggest bathroom. Once in there, have him stand on the toilet seat with the lid down. Take off his pants. Tear the pull up off at the seams (some have velcro, makes it a bit easier, but it's pretty easy, anyway.) Bring a plastic bag of some sort with you, toss the poopy pull up into the bag. Then, have DS bend over and put his arms on the wash stand/counter and it should make it easier to get the poopy cleaned off and also make it so you don't have to find a way to lay him down and/or bend down onto the non-existant floor!