Toffees boss David Moyes signed him as a 16-year-old from Oldham but he failed to make it into Moyes' first-team plans and was loaned to Macclesfield.
But following his exit from the Dale, Spencer has since spent a successful trial spell at Roots Hall and Shrimpers boss Tilson is happy to have captured him.
Tilson told the club's official website: "He has a good appetite and impressed in both the game he played in with the reserves and in training.
"He's hungry to do well and we've given him the opportunity so it's worked well both ways. It's certainly worth the punt."