One of our children will be flying to the UK with his grandparents a little ahead of the rest of us. I know in our situation, we are getting a notarized letter with everyone's signature (mom/dad/grandma) stating who he is and that we are allowing our son to leave the country with grandma/grandpa. We will then give copies to them to travel with, fax to airline in advance, etc.
I know your situation is different since he may be flying alone, but I would put together a notarized letter with both parents signatures stating you give your permission, who he will be meeting at airport, etc. Make a few copies and send to the airline too.