I'd definitely mute most chat tabs, except game stuff like NPC dialog. Most content Blueside seems pretty safe for kids, less so Redside (there are some missions that have heavier themes so it might be harder for kids to process). My biggest concern would be other users but that should be ok if you're supervising him. I've seen costumes that could raise some eyebrows but they're not explicitly NSFW.
I don't know if you've talked to him about people on the Internet, but I'd do that. Explain that he shouldn't give out his personal information, though I'd also recommend telling him that if someone says something weird or scary to him he can talk to you (or if he sees something he doesn't understand or any upsetting content). CoH is probably a relatively nice place to get the hang of these basics, as far as things go.
I see some people introducing CoH to their kids and teaming with them. I hope it's still around when or if I have kids, it sounds really fun. Maybe CoH parents could talk to each other and make a group for them and their kids... the world's most heroic playdate!