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.
Pressure - regarding the lead up to the D Day Landings


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