I would find out what troubles him about flying, rather his parents should, if you can't calm him down I wouldn't make him go against his will, he will have a real phobia if they did that. It is wrong to just dismiss their concerns because they are a child, in a few years time when he sees friends and family flying off, he might decide to try it, if not there's other ways. If he was my grandchild, I would stay with him and have lovely days out or take him to the seaside by train, the rest of the family can have their holiday and everyone happy, couldn't see him upset.