...Do you have any thoughts on dealing with issur and challenge of Hotzaat Zera LeVatala with high school/ yeshiva boys (either individual counselling or on the agenda of mussar-work a rebbe might do with his shiur)?
Yes. What I did was to use the piece in Shiurei HaRav that RYBS has about Negi'ah (I used to have the book, but lost it several years ago - it is a remarkably accessible piece about how Judaism is not meant to make you happy but heroic - it is a great segue into Misnagdus vs.. Chassidus, which can also be incorporated into this discussion - see further).
I then used the MME Kuntres HaChesed on Ahavat HaMin - but in requires contextualizing - i.e., framing the whole conversation of the tachlis of the Beriah and of Man specifically (Tzelem Elokim, V'Halachta B'Derachav). I then explained how being a Ba'al Chesed is meant to lead to Happiness, and that Chassidus does believe the goal is Happiness.
I then explained how HZL is the ultimate act of selfishness, and contrary to the idea of Chesed that is at the core of our existence and the purpose of sexuality.
My experience is that boys generally accept the logic (they find it refreshingly profound relative to what they usually hear), and that on occasion they will even tell you they have decreased their HZL. I think that is the best outcome to which one can aspire.
This even works with lower levels...