I'd say you might let him try, since it's a wonderful hook for adventures and roleplaying his relations with his mentor. But you've got to make something to avoid a totally overpowered combo. You've got multiple options : the mentor may fool him, allowing him to get a false name, but acting like it's the true, and then punishing him during possession in the way it wants, to make him learn the lesson, or he may make a pact with the spirit, with a heavy price as said Gun Nut, or something else.
But assuming a Mentor spirit must have an huge Force, the concept of the summoner who can summon normal spirit, send them like drones or personnal army, AND get the huge power of possession - attributes near to the max, immunity to normal weapons and so - is really overpowered, don't allow it.