Any tips or recommendations for talking to your 15 year old son about certain discreet matters? Perhaps a book I could give him or what I should or should not say?

I would start by emphasizing that all deliberately willed and completed impure acts are mortally sinful.  Dwelling on impure thoughts (which cannot be helped if they arise spontaneously, it's what you do at that point which determines the sin) and, obviously, reading pornography, are also to be avoided.

I always took the approach with my son (now 18) of explaining certain urges and manifestations at some point before they would become an issue, explaining what happens with the changes in one's body and what to expect when it does happen --- no surprises.  This worked very well.