I could be wrong, but think the fruit he is referencing are the people of the day. However, fruit worthy of repentance are people who really acknowledge and are willing to follow the proper path and are genuine and sincere about making reform.

Those who are willingly defiant are considered unworthy fruit, but remember all things are possible through GOD, and with JESUS, even the robber who was cruicified with him was saved because of his repentance.