Being a Death Knight tank and recent applicant (Now raider) of my current guild I know where this guy's coming from but he's taking it too far. My GM is also one of the guilds "Main Tanks" and we have a good laugh about me pulling aggro from him on purpose during trash pulls, him being a Paladin and all.
My advice: See what this guys attitude is like. If it's playful and fun, he's just trying to impress and socialise with the guild. If he's generally quiet and keeps to himself or brags alot, he's probably doing it just to be a dick and undermine you.
"God grant me the serenity to accept the things I cannot change, courage to change the things I can, and the wisdom to know the difference." - Reinhold Niebuhr