The 21-year-old will remain at Roots Hall until the end of February and could make his debut in the weekend's clash with Oxford.
The midfielder has made six appearances for the Saints this season and scored in the Capital One Cup victory over Stevenage in August.
His last appearance came as a late substitute in the 1-1 draw with Norwich in November.
Shrimpers boss Paul Sturrock told the club's official website: "We're delighted to have signed Ben. He's someone we identified as a player we feel can help us push for promotion.
"He has played in the Premier League this season so it just shows how highly the young lad is rated. He will offer us something different.
"He's here initially for 28 days and we will see how things progress during that time."