Michael Slaven, 23, from Dumbarton, went missing in the early hours of Friday.
His devastated family, including parents John and Elaine, were joined by friends and members of the public in searching the area around St Enoch Square.
Mr Slaven, 52, said: "I haven't lost all hope of finding Michael, but it is becoming very difficult.
"It's getting to the stage now where it is hard not to find someone who hasn't heard about Michael going missing, which is good.
"But we are no further forward and haven't had any new information.
"It leaves us wondering what else can we do to find him?"
Electrician Michael had been with friends before leaving Sugar Cube nightclub, in Queen Street, alone.
At around 2.30am he was spotted in CCTV images taken near St Enoch subway station.
His parents said he appeared to have been acting erratically in the images, raising fears that his drink may have been spiked.
Around 80 people gathered at the station yesterday, as two searches took place in the city centre, at midday and 7.30pm.
Mr Slaven, an estate agent, continued: "We have been searching all over, but we are no further forward.
"We're concentrating on a half-mile radius from where Michael was last seen, as I believe he is still in that area.
"There has been around 20 minutes of CCTV tracking Michael in that area, but nothing since.
"Surely a camera would have picked him up if he left that area?"
Michael is 6ft tall, with a slim build, clean shaven, with short brown hair and brown eyes.
When he was last seen, he was wearing a white T-shirt with a grey square emblem on the chest, olive green chino-type trousers and dark plimsolls.
Michael has not used his mobile phone or accessed his bank account since he was last seen, leaving his family fearing the worst.
Yesterday police confirmed that a man seen a at St Andrew's Cathedral, in Clyde Street, in the early hours of Friday was Michael.
It is believed he climbed over a fence - possibly injuring himself - to get into the car park at St Andrew's Cathedral.
A spokesman for the Archdiocese of Glasgow confirmed CCTV from the area had been passed to police.
Mr Slaven said he is "positive" the CCTV images from St Andrew's Cathedral show his son.
He said: "He could still walk, but looked like he had maybe hurt himself. His arms looked limp."
Police Scotland said "extensive Inquiries" to find Michael are continuing. Anyone with information is asked to contact Dumbarton Police Office on 101.