Can someone suggest an appropriate online social network for a 14 year old where he can discuss his views without much fear of being exposed to an inordinate amount of age inappropriate material?

My nephew is 14 and is having debates with cousins on Facebook about religion. He’s an atheist, and his cousin is a fundamentalist Christian. It’s starting to worry his parents that this debate is going to blow out of proportion and cause a rift in his family. They would like to point him to an appropriate social network where he can safely debate his views. Do you think Fluther is appropriate, or is there a more age appropriate forum?