Counsel and advise him; if he refuses to listen, then let adversity teach him.
Give advice first, as you should. But if words don't get through, let the consequences do the teaching, since they teach whether a person wants to listen or not.
It is a light rain that chases a child indoors; it is a raging torrent that shakes the raffia palm to its roots.
What sends a child running indoors is nothing to a grown palm, yet the palm has its torrent too. Hardship is measured against the one who bears it, and everyone, however strong, meets a force that can shake them.
Even if a log stays ages in the water, it never becomes a crocodile.
Time spent among a people, or in a place, does not turn you into one of them, any more than a log left in the water becomes a crocodile. One telling gives it as a light-hearted answer to an outsider who is told they have "become a Fulani".