Is the kid scared of planes?
If he is, I'm not sure what will help. I don't feel particularly fearful of planes. Perhaps asking why he feels fear of plane journeys.
Perhaps to explain what's going to happen on the plane and then to explain what's happening as it's happening would help(ex. "we're waiting for the plane to move" "the plane is going into the air" etc.). Also, if one parent is afraid of flying, have that parent sit in a different place with the other kid/kids, so that the fear isn't picked up so much.
One thing I do know, they need to be sure to have sucking sweets to reduce the pain/discomfort of taking off and landing (ears popping can really hurt sometimes). Sometimes it doesn't hurt at all but best to be prepared.