He could be married with kids and serving in the UK infantry at that age!

No particular documents are needed, it's more about whether this is going to sound plausible to the USA CBP in terms of him respecting the terms of the ESTA. So some documents indicating why he would need to leave the USA in due course would be helpful (e.g. to resume studies), along with full details (addresses, phone numbers) of the relatives would perhaps be helpful. It's not so unusual for 17 year olds to be travelling solo so I doubt there would be a problem.