The teenager joined Hoffenheim on loan in the summer and has since scored six goals in 15 appearances.
And academy director Mertesacker has praised the Englishman for making a brave call.
"When you compare the Bundesliga to the Premier League, there are similarities but there are differences.
"At the moment the Bundesliga suits him, and he went to a club where they are used to bringing up young players, educating young players and giving young players a chance in their first team. He has a young coach who will educate him and give him an insight.
"Reiss is getting out of the comfort zone he was in at Arsenal, so the loan system should work brilliantly for us.
"Through scoring those goals, he gained trust from the coach, from the fans, by learning some German and earning the right to start and make an impact.
"I think he has done a brilliant job but that all fell into place because he wanted to go."