One more thing...put your mindset in the position of partnering with him as a guide.
You can ask straightfoward questions...
"Have you noticed that your penis changes? "(yes)
"That's called an erection, I have them too. "
See????
Let him identify with you as the older male guide.
Get it???
Let Mrs. Gaz read this...she'll get it.
Editing: I think if you want to add something of a religious nature to the discussions, it would be appropriate to use the verse about being "fearfully and wonderfully made". (Ephesians?) I think that's a great message for a believer to transmit to their kids.
I also think that somewhere along the way, you might consider (if you think it fits) that you send the message that emotions are involved along with the physical stuff...responsibility to each other to care for emotions and reasons to live by the Law of Chastity. Real practical reasons, not just the sin/condemnation reasons, Gaz. I'm thinking in terms of messing up one's potential so far as pursuing education, having a good life style, living conditions. It's all tied together.
Okay...I'll tell you a story. :-)
When my kids were younger I sometimes spoke of an Aunt of mine. She had a kid "out of wedlock" just after high school, no husband at that time, went on to have various relationships with/without marriage, ended up having a small ton of kids, on welfare, eating canned soup, and the last time I saw her, she was living in a junkyard.
My question over the years in conversation with my kids was
"Do you want to live in a junkyard or do you want to have a good life for yourself?" 