If he wants to be paid as an offensive lineman, that's fine. He has no right, though, to demand guard money, if he doesn't want to be categorized by one position; otherwise, he'd be categorized as a guard.
The idea of paying him the average of guard/center, then, is a fine idea, but the agent obviously wants more than that. It's a nice attempt by the agent to gouge the team (or whatever team) for more money, but there's very little logic involved.