If the plans are still available then yes, it can easily be done. If not then the only option would be to swap out the SIM cards between your phones. However, his number would then be your number and your number would be his new number.
If you switch the SIM card, yeah. But I believe you don't want to switch number neither. Usually you can't switch plans, but maybe if you switch SIM card and the numbers? I don't know if such thing is possible