Find stories of Gedolim that will impress upon him the severity of the issue at hand. Inadvertently and subconsciously have him realise that we are of the higher “club” with proper moral standards together with these Gedolim.
On SEED I was put in the common awkward situation of having a woman’s hand outstretched to welcome me with a normal social handshake. Politely and cautiously I told her that the only woman’s hand I shake is my wife’s. A bit taken aback she asked why. I bluntly told her that many poor-taste relationships have started by a public casual handshake, with a little extra squeeze or smile or wink that led to other future contact. An undeniable truth in today’s society. She immediately turned to her husband and said that is the most beautiful thing I’ve heard, why don’t you do that too?
Obviously each situation must be judged with saichel. Sometimes one must turn to the maikilim in such matters. However the point here is that as far fetch and extreme as shomer negia might sound NO ONE will argue that that is how immoral relationships begin.