David Dixon texted his aunt saying he was safe after the two airport blasts, but his family fear he was caught up in a further explosion at a Metro station.
Mr Dixon, from Nottingham, lived in the capital with his partner Charlotte Sutcliffe and their son.
He is thought to be the only Briton still missing since Tuesday's attacks.
Downing Street said four Britons were injured in the blasts, which left 31 people dead.
Mr Dixon's partner told the Telegraph that police have his dental records and are seeing if they match any of the victims.
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His friend Simon Hartley-Jones said the couple had lived in Brussels for about 10 years but still had a house in Nottingham and came back regularly.
"David is a programmer and works in the centre of Brussels," said Mr Hartley-Jones. "He makes his way there every day using the Metro system.
"For some reason he hasn't made it... and we're really concerned about him.
"It's really unusual for him not to contact [Charlotte] - it's really out of character."
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He described Mr Dixon, originally from Hartlepool, as "a lovely guy" and "an amazing man who deeply, deeply loves his son".
The couple's son was staying with friends and was unaware his father was missing, he added.
"They still have a house in Mapperley [in Nottingham] which they come back to regularly, this is their home.
"They were supposed to come back this weekend, all the family were due back this weekend to spend Easter with us here, and we're missing them deeply."
Mr Hartley-Jones has appealed for anyone in Belgium who might know of Mr Dixon's whereabouts to get in touch.
"Charlotte is just desperately looking for David and we need any information you can get," he said.
"Please just check in hospitals in Brussels, just check that David is not one of your patients, and just let us know."
Ms Sutcliffe's sister, Marie, said they were hoping to hear from a hospital or police that he had perhaps been injured but was safe.
"Not everybody has been identified yet among the injured, so it's just waiting for that process to happen," she said.
She plans to go out to Brussels to support her sister. "Understandably she's very, very distressed," she said.
"It's just waiting, which is heartbreaking and very worrying."
Mr Dixon's cousin, Philip Dixon, said his family was "fearing the worst".
"He takes the Metro to work and unfortunately it's looking like he could have been on this Metro train at the time of this bombing yesterday," he said.
"Charlotte tried phoning his work and he didn't turn in for work yesterday morning so we are fearing the worst at the moment."
Downing Street said four Britons were confirmed to have been injured in the twin blasts which hit Zaventem airport at about 07:00 GMT and Maelbeek Metro station an hour later.
So-called Islamic State (IS) has claimed responsibility for the attacks.
A No 10 spokesman said: "We are concerned about one missing British national and we are in close contact with the Belgian authorities.
"We are aware of four British nationals who were injured in the attacks - three are being treated in hospital, one has already been discharged.
"Our embassy staff are working to assist all British nationals affected."
Mr Dixon has been working as an IT contractor for securities settlement group Euroclear, which has an office a few Metro stops away from the station that was attacked.
Tristan Peniston-Bird, a Euroclear spokesman, said: "We confirm that David Dixon is a contractor working for Euroclear in Brussels, who remains unaccounted for following yesterday's tragic events.
"We are in touch with his partner and continue to co-ordinate with the authorities in locating David.
"Our primary concern is the safety and protection of our people, and our thoughts go out to those affected by yesterday's attacks."
Friends of a computer programmer missing in Brussels since the bomb attacks in the Belgian capital have said they are desperate to find him.