Liam Colgan has not been seen since he left his brother’s stag party in the early hours of 10 February.
The 29-year-old walked off alone into the city, with CCTV capturing his last known sighting outside a Hamburg bar.
His brother Eamonn was due to get married on Friday, but the ceremony has been postponed while the search continues.
He said it was possible Liam could have travelled beyond Hamburg.
Speaking to BBC Radio Scotland’s John Beattie Programme, Eamonn said Liam’s family and friends were trying to get their appeal for information spread more widely in Germany.
He said Hamburg police may even be “fearing the worst” that his brother has not survived the ordeal.
“We obviously aren’t from that standpoint,” he said. “We still have a very strong belief that he is out there.
“For some reason he has not been able to be in touch with us.
“There are so many theories that are going through your head. Our latest is that Liam sustained some sort of head injury that has confused him as to who he is and where he is.
“He might not even be aware that he is actually lost.”
Eamonn added: “There is a strong possibility he is no longer in Hamburg and that is why we are trying to raise awareness as far and wide as we can.”
He admitted to initially feeling “mocked” by Hamburg police, who regularly encounter cases of people going missing temporarily after nights out in the city.
The family had protested it was not in Liam’s nature to disappear partying for a number of days.