The best way to aproach this is by laying down some simple rules.
Without making it a big deal, tell your son tht he is getting a big boy now, and he will hear stories about mums and dads from people at school, and he may not understand. Tell him that you will always tell him the truth, and if he has any questions at all, he must not be shy about asking you, and it's nothing wrong or dirty, it's part of growing up.
Then wait for the questions - and be honest, within his ability to understand, at this age, keep it nice and simple.
The important thing is to build a feeling of openess and lack of embarassment between you - which may be easier for him than for you, but you need to set this in motion now - for when he is older.
I certainly wouldn't sit him down for 'the talk' as the previous generation did - you'll probably scare him silly!
Be ready for his questions - and stress that this applies in all areas of his life - he can talk to you about anything - and hopefully he will.
Good luck.