All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The correct way of discussing with him about this issue is to tell him that a person passes through many stages in his life. Among these stages, there is a stage when a person is held accountable for his acts, so he is obliged to perform obligations and refrain from prohibitions, as he will be held accountable for being negligent and he will be rewarded for doing good. There is no harm also to mention to him that when a person reaches the age of puberty, he becomes pubescent. If you consider that it is better to make him listen to a tape on this subject or give him a book to read on this subject, then it might be that he will benefit more from that, Allaah willing. May Allaah bless your son.
Allaah Knows best.