Friends of Alan Drennan have claimed the 21-year-old sustained head injuries when he was beaten by officers after arriving on the island on Saturday.
He was found dead in his hotel room on Sunday, less than 24 hours later.
A solicitor for the family is speaking with Spanish, British and Irish authorities to establish how he died.
Mr Drennan Jr was a mechanic and doorman from Newtownabbey.
His family now have "more questions than answers" over the circumstances that led to his death.
Mr Drennan's father, also named Alan, said "what happened to Alan and our family is every parent's worst nightmare".
"We want to find answers not just for us but for everyone else's child who goes abroad on holiday and particularly to Ibiza," he added.
Mr Drennan Jr's friends have alleged he was beaten by Spanish police after being taken into custody at the airport in Ibiza after an incident on the flight to the Mediterranean island.
On his release, he and the 10 friends with whom he was on holiday went on a night out.
He had been staying at a different hotel to his friends and had returned there afterwards on Sunday morning.
His father said his son "hadn't even his case opened" when his body was found in his room later that day.
Madden & Finucane, the law firm representing the Drennan family, said they were in "protracted discussions" with authorities to ensure the "precise circumstances" of the death are "clearly established".
The exact cause of Mr Drennan Jr's death is unlikely to be established "for a number of months", the firm added, due to the timescale for the availability of a toxicology report.
"Alan's friends have raised a serious allegation that he was assaulted by Spanish police officers prior to his death causing injuries to his head and upper body which were clearly visible to his friends upon his release from police detention at the airport," Madden & Finucane said in a statement.
"We are assisting the family in trying to establish the truth of all possible contributory factors which led to Alan's death."
The Police Service of Northern Ireland (PSNI) has said it is assisting Spanish police in the investigation into the death.
Supt Kevin Geddes said the PSNI was "gathering any relevant information and speaking to any potential witnesses" to assist in the probe.
The father of a County Antrim man found dead in Ibiza wants answers after allegations were made that his son was beaten by Spanish police.