"You are in the Brotherhood." It wasn't a question - where Kurt came from (or when, actually) Magneto's group was a terrorist organization fighting for mutant rights, with full intentions to end humanity. Even if they didn't come out and specifically say it.
Sighing, he thought of how to best formulate his answer. "I...I can't say I agree with Magneto's ideals." Or what will eventually become his ideals. But since Magneto was here in the school now, would he stay? Were they changing history by messing with time? Deep questions. "I know that humanity hasn't done much for me - hasn't done much for us," he added, placing his hand on his mother's again. "But I still believe in the Professor's vision. Now that you know, will you..."
He wanted to ask if she would love him anyway. But didn't do it. Maybe because he was afraid she wouldn't, and he desperately wanted her love.