The 19-year-old has scored four goals in six games on loan at the League Two club – and he has stated that he wants to stay.
"I don't mind staying. I am a Southend player at the moment and I'll speak to the gaffer about what happens next," he told the club's official website.
"I want to play football, score goals and get 90 minutes under my belt each week so this is a good place to do that."
Assombalonga has also been linked with a host of Championship clubs.