Imaro
Legend
With both hands on his halberd/glaive/pike he's going to have a hard time using material components. Eldritch blast is going to be fine, but something like Hold Person would not. AoO on approach in exchange for being unable to cast a sizable portion of your spells is a fair trade in my mind. If he tries taking a hand off his weapon, then he's no longer wielding it, and loses his AoO (I'd let the player know this ahead of time).
I think the warcaster feat allows him to cast with a weapon in both hands... of course I agree with many of the other posters, it doesn't seem like that powerful of a combo for the price...