The Metropolitan Police is offering a £20,000 reward and calling for "the wall of silence" around his disappearance to come to an end.
Mohammed Shah Subhani, 27, from Hounslow, west London, was reported missing shortly before midnight on 7 May - around 12 hours after his family last saw him.
Mr Subhani has not used his bank account or mobile phones since then, and his white Audi Q3, with a registration LC67 CXS, has also not been seen.
Detective Chief Inspector Noel McHugh said: "Mohammed, who is also known as Shah Khan, is incredibly close to his family and is only a few months away from becoming a father for the first time, so clearly his absence is having a devastating impact."
He added: "We believe there are a number of people who may have information which could assist the investigation; but they are staying silent."
Mr Subhani previously worked as a courier, but police believe that after losing his job he may have begun to deal cannabis.
It is believed Mr Subhani was in debt. On the day of his disappearance, Mr Subhani was expecting to receive almost £10,000 from two sources.
Mr McHugh said: "Mohammed should have been in possession of at least £9,800, if someone knew this it could be significant.
"I believe that Mohammed has got out of his depth in some kind of criminal activity.
"At this time there is a wall of silence and people are in the know, we need those people to come forward and tell us what they know."
The £20,000 Met reward is being offered for both information leading to the whereabouts of Mohammed, and also information leading to the identification, arrest and prosecution of anyone connected with his disappearance.
Mr McHugh added: "This is an investigation where I hope I am wrong, but I fear that Mohammed may have come to some harm.
"From meeting his family and scrutinising his lifestyle, it is apparent that all of his routine activities have stopped. For me the disappearance of his car is astonishing, it could well be on false registration number plates and may well have been parked up.
"Mohammed hasn’t contacted his family, which is unheard of. They have described that he is a carer for his ill father and he was elated that his partner is pregnant."
His sister Quirat Subhani was due to get married in eight days' time but has cancelled her wedding because of her brother's disappearance.
She said: "My heart hurts to think what could've possibly happened to my kind hearted brother, he is such an amazing, soft spoken, kind hearted, respectful and loving person. I am hoping God will return him back to us safely for my mum and dad's sake, for his unborn baby’s sake.
"However I believe two things, I think someone might be keeping Shah just to prove a point or somebody might have hurt him and they might have dumped him, if someone has killed our Shah, we deserve to know.
"I am praying Shah will come home and somebody out there will help us, I’m begging the public for help. Imagine it was your son or your brother, imagine your parents going through what my parents are going through.
"Please just pick up the phone and get in touch with the family or the police."
:: Any witnesses or anyone with any information is asked to call police on 101 or contact via Twitter @MetCC quote 19MIS017114. Contact Crimestoppers anonymously on 0800 555 111 or online at crimestoppers-uk.org.