Tell your warrior if he wants more uber gear than what's he is wearing, respec protection for a month or two. Get all the gear from all the instances (and heroics), then go crazy with the new epixxx. And the faster he does this, the faster he gets the gear and the less time he needs to spend in Protection.

That is not to say Arms/Fury is not good, but from your description he's not exactly the sharpest tool in the shed (you mentioned the Shadow Priest thingy). I've known many warriors who can tank very well with very few points in Protection (if any).

Make sure tho that you're also seeing his view. "Why should I be protection if the priest is shadow?" could be another reason. Be prepared to do the same if he asks that "you jump I jump" in respeccing.

Another factor is that the warrior could be his only toon. A Protection Warrior farming for gold & mats is like visiting the dentist. Offer to help him farm as another argument for respeccing.

Thus the fastest route for him to get gear is from instances, and the easiest way to do it is with min/maxing your group (pure tank, pure healer, and 3 dps/cc/support).

If he insists on being what he is, tell him that his playstyle is PVP, and that he can forget about grouping with you (or any other people) for heroics and such; and he can spend the rest of his WoW days doing AB, WSG and AV (teh horror!).

You're better off looking for a druid bear (who can farm, pvp, tank & dps all in the same talent tree).