This is a great question. Let me name a few of the ways in which Telemachus does not fill Odysseus's shoes.
First, Telemachus is a youth. Therefore, he does not have the experience, strength, and courage to take on the suitors. All he can do is watch. In many ways, he is helpless. His very name suggests this. His names comes from two Greek words. "tele" and "machus." The first word means distance and the second means battle. So, we can say that Telemachus is one who fights from afar. Again, he has little experience.
Second, Telemachus does not have one of the great traits his father has. Cunning. What make Odysseus special is not merely his military power, but his cunning and ability to be strategic. He is a man of many guises. Telemachus cannot fill this as well. So, what Telemachus does is to leave to find out more about this father. In the process, he grows in courage and by the end of the book, he become a man. However, he still is no Odysseus.