check the nvc website to see if his priorty date is current or not if it is then he can apply for adjustment of status 485,765,864 and all that good stuff all u need to do to upgrade is to write a letter asking them to upgrade ur petition and explain ur situation that ur mother is a usc now and dont forget to sent a copy of the citizenship certificate upgrade is not necessary but to be on the safe side u could do that or u could just go ahead and file for AOS and they will take care of it but if u do it it may save u some time, goodluck.