The staff will be happy to watch him while you go the loo once you are ON the plane.

I would pop a blanket in your bag, and go to the disabled toilet, pop the rug on the floor and they can sit on that while you go to the loo I have done it many times! lol

I have some carriers here you can borrow if you don't have one, but they are invaluable for solo travelling with kids from my experience.