I would agree with RachieBaby... I'm not a Mum yet, and I think each child is different, but if he's asking the questions then I think he deserves the honest answers. You shouldn't have to go into your sex life but just be blunt. My Mum thought she did a good job as she explained how babies were made when I was 3 and she was pregnant with my little sister, but funnily enough I'd kind of forgotten about it by the time it became relevant!! Ok, obviously I'd figured out how babies were made, but it was the only talk she ever gave me, apart from telling me when I was 14 that she'd send me away if I got pregnant.. charming, it meant I never discussed anything with her ever again. She says now she was just trying to scare me and never meant it, but it meant I felt really alone.
I remember that I'd never heard of oral sex when the "subject" first came up, and I was 13 or 14. I ended up going along with it because I didn't want the boy, who I really liked, to think I didn't know what it was
Maybe I was just super super naive, but if I'd been better informed I like to think I'd have known how to say no.
I don't know what sex ed classes are like in the UK nowdays. I remember we had a talk about periods when I was about 8, but again I was still shocked when mine arrived when I was 10, guess I thought it was something that happened to adults. We had sex ed, which involved a couple of half hour classes watching some nudists play tennis... not quite sure what the aim of that class was!! I'm sure just having books around and it being something that your son feels he can approach you about will be no end of help. I can imagine it's embarassing for you but I bet he'll thank you for it, good luck!