I have a player on my server that 'seems' to be stuck in Creative mode, but he can receive damage. I have tried changed his gamemode to survival several times and it doesn't seem to work. Can anyone tell me if there is a mod or a cheat for this?

He says he's not using a mod, but I don't know, none of my other players have come to me about this and most of my players I've known for a while. He is the only one with this problem and I find it strange that he isn't in full on creative mode, because he still dies by creepers.

If anyone can help me or give me some ideas on how to fix this or maybe tell me of some cheats he might be using I would really appreciate it!

~Daenerys2012