McCallum is on-loan at AFC Wimbledon and penned a deal to 2015.
He told whufc.com: "It's been a good few weeks. I have signed a new contract with West Ham and been doing well and scoring goals, so I cannot ask for any better.
"I feel like I am physically fit and when I scored two goals in the opening Under-21 game of the season [in a 3-1 home win over Reading], I felt I was ready and it gave me a lot of confidence to go out and play my game.
"I would like to thank all the staff at West Ham, particularly the physio Tom Smith, for getting me right for this season. I only played one game last season, which was frustrating for me and knocked my confidence.
"I had a good pre-season and those two goals in the opening league game brought my confidence back."