I guess i'm in a position to give you pretty much first hand (excuse the pun) advice on this.

I'm 16 myself and started at the age of 12, my Mother was always strictly religious and sex was a no-go subject for her and my Father never seemed to care that much. My mother talked to me for the first time in my life about sex and sexuality a few months ago, I was slightly annoyed to be honest at the lack of advice and support I had been given(or lack of).

After the conversation, she left me feeling very embarassed and insecure, so I would suggest you try not to sound nagging or aggresive.

One of the things I find most annoying is the lack of privacy I get, so i'm sure your son feel would savour that.

Good luck