The 20-year-old, whose initial deal was due to end after Saturday's clash with Plymouth has excelled in his four games for the Blues, playing a vital role in their recent run of fine form which has seen them win their last three.
"Billy typifies what we're trying to do here," boss Gareth Ainsworth told the club's official website.
"He's settled in and got into the spirit immediately, and the way he plays makes it evident for all to see just how much he wants to win and how much he wants to be a part of this.
"I had no hesitation in extending his loan once Sunderland agreed. I'm very grateful to Sunderland for allowing us to have Billy here for longer.
"Hopefully we can improve him as a player and he can improve us a club, that way everyone's a winner."