Agree with the others- he may already be active, and pretty aware. I'd sit down with him, and run through exactly what he is aware of or not.
"So- what do you know about contraception? Well, of course you should always use condoms, but if you need a back up method, what do you know about x,y,z...?"
"What do you know about STIs... well, yes those are the obvious ones... but what about x,y,z?"
Also, maybe a bit of emotional stuff- as I was reading another thread where an OPs son was devastated about being 'dumped' during GCSE time- and maybe it would be worth talking over some of these sorts of things, and making sure that he knows that the communication channels are always open.
Just be yourself, and that should avoid embarrassment. I think that there is no way that you chatting about sex with him will 'encourage' him! Teenage hormones will take care of that either now, or later!!!