The safest answer here would be that he can only get as much from Erik as he wants to allow him to get. He has his helmet with him, which effectively renders him a psychic blank (err ever since Marvel retconned it into doing that, anyway). And even when he's not wearing it, he has some of the best psychic defences of anyone who isn't a telepath themselves. I imagine that trying to poke around in his mind would be like hitting a metal wall, though iirc it hasn't been specifically described. He might be able to push his way in there if he's powerful enough, but Erik would definitely be able to notice the intrusion by that point.