Tom Armstrong, from England, had been on a dive master course on Koh Tao since last April, but was due to return over Christmas, the East Anglian Daily Times newspaper reported.
However, his mother told the newspaper he had postponed the journey back after “running into trouble” in Bangkok, and was instead due to catch a flight home on January 8.
But Mr Armstrong never boarded the flight and has not been heard from since. Police have launched an appeal for anyone who can help discover his whereabouts.
His mother, Helen Armstong-Bland, told the Daily Times that the family were desperate to hear from him.
“He was due to come home before Christmas but for whatever reason didn’t arrive,” she was quoted as saying.
“That sent alarm bells ringing, but we received an email from him saying he had run into some kind of trouble in Bangkok.
“We spoke to somebody at the dive centre to see if he had returned there okay and they informed us by e-mail that he had booked a new flight to return [to Britain] on January 9. But we did not receive details of this so we waited to hear details from him when he landed back in the UK.
“We waited for the last train from London but he didn’t arrive. That’s when we spoke to the police.
His mother said the family intended to travel to Thailand if there continued to be no contact from her son. She added: “We all really miss him and love him and want him to come home.”
Mr Armstrong has a younger sister and a younger brother in the UK, and an older brother currently living in Japan.
The Daily News said police in Britain said they were growing increasingly concerned over the whereabouts of Mr Armstrong, who had been in contact with his family through email and Facebook until his intended date of departure.
Officers are appealing for any information that would indicate where he is, and that he is alive and well.
Mr Armstrong is described as about 188 cm tall, of medium build with fair, slightly wavy medium-length hair. He sometimes has a short beard, which is more red than fair in colour.