The England U-21 international will remain at Elland Road until the New Year but won’t be permitted to play against his parent club on Saturday.
Simon Grayson told the club website: “I would like to thank Brian McDermott at Reading for allowing Alex to stay on here for another month.
“He has done really well for us in the time he has been here, he’s a fantastic young goalkeeper, and we’re grateful to Reading for allowing him to stay on."