The 23-year-old striker, who has been training with the Shrimpers all summer, has penned a one-month contract at Roots Hall.
Boss Phil Brown told the club's official website: "Don provides us with something that I feel we're missing a bit and that is an abundance of pace.
"It's great to have him on board and I'm sure he will do well for us.
"He's not been match fit in the game he's played so far but he's getting there and I'm sure he'll excite fans."
- Sports & Recreation