No boy is an island. Where are his parental/guardian people in all this?

You could either be heading for becoming a favourite unofficial uncle who's always remembered as one of the highlights of this boy's growing up, or to falling under deep suspicion and suddenly finding that the boy's open arms are receding into the parental shadows, never to be seen again.

What you can say to him relates entirely to this factor.

Only go the uncle route if you're sure you will never have a weak moment in relation to sexuality with the boy. This means really deeply understanding why you wouldn't be able to do it even in the unlikely event that he seemed to want you to. One bit of advice some people overlook: never drink or take any drug when you're in this situation. You don't want to find out about unexpected impulses by having them take you over, as happened to one young BL I know recently (who has already had to explain it to a judge).