The centre-half, 19, has impressed manager Mauricio Pochettino with his performances for the club's Under-21 side.
Pochettino told the club's official website: "I have watched a number of the U21 games in recent months and have been particularly impressed with how Jack has fared.
"He shows great leadership qualities and commitment in his performances. He clearly has a lot of potential.
"Jack trained with the first team during the recent international break and fitted in with the group extremely well."