@Alasdair-Scott0 No, you can't book for him, third party bookings are against Airbnb's Terms of Service. The person who books has to be one of the arriving guests.
It's great that he wants the experience of staying with locals. What you could do is message hosts of places he is interested in, explain the situation, get your son to introduce himself to the host in that message in a way that will show he's a good, responsible person, and then he could get back in touch with those hosts when he is 18 and sets up his own account. Hopefully some of those places will still be available.
He could also check out Couch Surfing and Woofers.
And I hope you've trained him well as far as cleaning up after himself and respecting others' property. Many hosts are leery of hosting young guests because they tend to be slobs who cluelessly wreck stuff and ignore house rules.