My kids are into Gaia in a fairly big way. You use a pseudonym (very good idea anyway) and it's fairly easy to remain anonymous and safe. However, if I am given the passwords (and it's in my kids interests to, so I can go on and get extra credits for them sometimes - being a helpful mum opens doors of information for me) than I can track whatever they're saying. Another option, if you don't have their passwords - shoulder-surf and at least see the name of their avatar (the pseudonym) then join yourself, under your own pseudonym. You can then 'converse' with your own kids, or lurk on the site to monitor their conversations on public boards, even if they're at school and you're at work.
This helped us a lot when easy child 2/difficult child 2's exBF broke up with her then blogged nastily all over Gaia ("she's got the maturity of a 12 year old and the bust size as well"). it gave us the open door to talk to her and counsel her as well as set some ground rules in communicating with tie guy, who is still good friends with difficult child 1. We could be there for her, because we found out about it all. I was able to plan to take her for some retail therapy, we bought a pretty, low-cut blouse and as she looked in the mirror she snorted, "Bust of a 12 year old, indeed! HAH!" And I was able to stop her blogging nastily about HIM, which would not have been healthy, or a good look.
So my suggestion - join the site yourself. Quietly. Then even when he's not with you, you can still keep an eye of sorts on what he's thinking. Because if he mentioned it to you, he's either already doing it or thinking seriously about it. He's 15 - most kids his age have pages like this.
Marg