Reeves scored twice in eight appearances after joining the Shrimpers on January transfer deadline day, including the last-gasp strike against Leyton Orient that secured a place at Wembley in the Johnstone's Paint Trophy final.
The 21-year-old completed his initial loan by starting last Tuesday's defeat at Barnet.
Work is underway to renew Reeves' stay until the end of the season, but he is only able to sign for another two months and boss Paul Sturrock is expected to take the May 18 League Two play-off final date into consideration.
A statement on Southend's official website said: "We are delighted that, to provide continuity in his development, Southampton have agreed that Ben will continue to train with us for the next two weeks before we look to extend the loan."
