An employee wants to stop his 401k contribution. Next time I cut a check for him, I can't just add that contribution amount to his check without taxing it right?

First is he allowed by the plan rules to stop payment or is he required to only make changes at certain dates, the plans rules vary.

But his payment to the 401 K appears to be pre taxed, so if that money is put back into his money, then the entire check amount is taxed. His new gross pay will be considered