The 22-year-old has linked up with the National League South side as they attempt to secure a play-off place in the final weeks of the campaign.
Willis has already had one loan spell this season, having previously spent time with Scottish side Queen of the South.
He made six appearances for the Doonhamers before an injury forced him to make a premature return to Teesside at the start of November.
He made his comeback in Boro‘s Under-21s side earlier this month, featuring in the 1-0 win against Liverpool.
He was back on the scoresheet in the Under-21s’ most recent game against Southampton, when he scored the decisive goal in a 1-0 win at the Riverside.
