Police believe the man was assaulted in the Mount Vernon estate but are working to establish exactly what happened.
He was found in the flat at Ross House at about 11.10 am on Monday.
The teenage boy, who was arrested yesterday in north Belfast, was released on bail earlier today.
A 41-year-old woman also arrested on Tuesday remains in police custody, assisting with enquiries.
PSNI Detective Inspector Pete Mullan said: “While we are following definite lines of enquiry and will be making make more arrests, I am again appealing to anyone with any information about this incident to contact detectives at Musgrave Police Station on the non-emergency number 101.
“Or if someone would prefer to provide information without giving their details, they can contact the independent charity Crimestoppers and speak to them anonymously on 0800 555 111.”