We live in a pretty open area. Part of the reason that it came up is that we took him to our local witchcraft store. My wife helped him pick out some stones that he had a connection with and let it slip that if he wanted to learn about Wicca, he could ask me questions, because I had been reading about it. The woman working there and I answered questions about wands, athames, and bolines that were on display. Of all places I could be, this is one where I could be pretty open.
His mom (probably) wouldn't be a problem. He told me that they stopped going to church because she didn't agree with what they were teaching, but she comes from a heavily Christian family in a rural area and they definitely wouldn't like it. My parents are also rural Christians, although looking back they gave me a lot of stones, charms, etc. that were for one thing or another. His mom can be unpredictable, so I like to avoid problems with her as much as possible.
The other issue is that I am a cop. I know there are pagan police groups in Europe, but here, police work is predominantly Christian, Catholic in particular. One guy calls me a pagan, because he thinks I am a Methodist. He just doesn't realize how right he is. :p
Honestly, I wanted my son to know, because it is a part of my life and I don't feel I should have to hide things in my own home. Now he's sad, because he won't be at my house for the fire at Litha. I told him that the reason we wanted to keep it a secret was that people can be afraid of people who they don't understand, and that a lot of bad, untrue things had been said about witches in the past. Then, I showed him out to do a reading with rune stones. After that, everything went back to normal.
He was probably prepped really well for the news. My wife meditates and he's meditated with her before. I made him a dreamcatcher that he has slept under for years. Grandpa gave him a buckeye for luck. He's also recently turned in to a big Harry Potter fan, so I had to explain why I am not a wizard. Overall, it went a lot better than I expected. But with all the witchcraft books in the house, he would have figured it out pretty quickly regardless.