As mentioned the alley was dark. So it is possible that there was someone else at the crime scene and the witness is mistaken to be that man. It is quite possible to see someone in the dark and mistaken them for someone else. As the alley was dark, eye witness testimony is not reliable.
The eyewitness doesn’t have any evidence whereas my client has an alibi and he was not present at the crime scene. Even if my client was standing near the boy it doesn’t prove that he killed someone.