I don't want to come off as being cruel, but there is such thing as
common sense. Ignoring Common sense is a form of "Natural Selection".

I'm going to assume we are talking about a younger guy here.

Young Guys generaly think they are indestructable. Their personal experience is usually that they can eat like pigs and still burn it all off. But those days come to an end, and the pigish habits can catch up like a raging flash flood.

when he can't button the favorite pair of pants any more - he might be more inclined to listen.