I think I'd leave it a couple of years. If he's frightened, he might be better to go with his parents the first time. It's a responsibility taking someone else's child on a plane (even if you are his great grandparent) and a greater responsibility if the child is frightened.
He may feel differently when he is older.
If you were going for some essential reason, like relocation or hospital treatment, I realise he would pretty much have to get on the plane but your circumstances are different.
Did you think you were poor growing up?
Why on earth would anyone plant.....
Only 50% of middle age adult manage more than 1 brisk 10 min walk a month.


I was merely pointing out that help is there, if people wish to avail themselves of it.