According to their sporting director, Max Eberl, the Bundesliga side have been impressed with the Hammers youngster even though he has only played just one minute of top-flight action for the side.
His only appearance came as a late substitute in October during a 3-1 win at Hoffenheim, but despite that Eberl says they would be keen to make the deal permanent should West Ham agree to it.
"He's a top lad," Eberl told German the Rheinische Post. "He's come on well and we're very happy with him.
"It takes time to find your feet in the Bundesliga, but we certainly want him to stay. West Ham now have the final say."