Most of the servers I have been to had ether a censor plugin or it was against the rules, but it still wond keep a child save, admins are not always online and cant always prevent people for saying bad things. But if he wants to play let him, dont worry he probably wont remember any of those words and if he does you just explain to him why he shouldnt use them and how aweful those guys are for typing those words.
I recommend Intercraften, it is family friendly and has rules that address swearing, griefing, stealing, and a whole lot more. It is a great server with a bunch of friendly kids and fun pvp arenas (capture the flag, infected, spleef, splegg, etc) (we do not allow pvp outside of the minigames). I am a moderator on there and I can guarantee a safe environment for your child to play in. We have a whitelist system, so you have to send in an application before you can log on. It's super easy, just go to intercraften.org and click on "join us" to fill out an application. We would love to have you join us.
Hope to see you on the server soon,