I think it is so great that you not only want to uphold the moral values of the Catholic church in your house but also want to give your son the WHY. I will never forget my mother telling me "Because I'm the parent and your the child"... well... sorry mom... but this isn't good enough. I am a person that needs the why! I'll do what you say... and follow your rules when I'm under your roof... but why insult me by not giving me the why?!?!
By all means explain why. However, as you may have noticed in perusing these very forums, even adults are quick to summarily dismiss that which is inconvenient for them or challenges their world view; teenagers even more so. If the child doesn't respond to reason, at some point the adult has the responsibility to play the trump card.