The boy was stabbed in Pretoria Road, Bordesley Green, on July 4, at around 7.30pm.
Police have since charged a 16-year-old boy with attempted murder.
Officers have released a picture of a person wearing a face mask and hat who they want to speak with as part of their investigation.
The victim was taken to hospital with serious injuries, which he is still recovering from.
A 14-year-old boy was also arrested in connection with the incident and has been bailed with strict conditions.
If anyone can help with the investigation you can get in touch with police by calling 101, quoting crime reference number 20/298057/25.