We had a question in my first game when the wizard channeled power one time.

He started off at equilibrium (4) power. He went into 1 reckless and channeled Power, and rolled well (> 3 successes and 2 boons). This caused him to channel an additional 7 power. This put him over his WPx2 threshold.

I couldn't find in the rules where it says when he must spend the maneuver+stress to contain the power (without venting). Is it immediately, or at the end of his turn (ie if he can quickcast to spend the power to prevent the stress/maneuver)? We thought it might be immediately after the channel power action, but the rules aren't clear about it.

I don't think it's immediately. If you do it when other manoeuvre's are done, in this case, at the end of the turn, it gives him time to blow off some power before he has to make the roll.