The 19-year-old has scored two goals in four appearances for the Dons, including the winner at Rochdale on Saturday.
McCallum's current loan spell at Kingsmeadow ends on January 19, but he has not ruled out extending his stay at the League Two club.
He told the Hammers' official website: "It's been a good few weeks. I've signed a new contract with West Ham and been doing well and scoring goals, so I can't ask for better.
"Whatever happens, if I return to West Ham or stay out on loan, I want to keep playing well and scoring goals.
"If I get the chance to stay with Wimbledon then I just want to be playing and making a name for myself in the Football League. If I come back, obviously I'll do my best for the development squad."