The man, who was arrested in connection with an alleged serious sexual offence in August 2006, had made six complaints about Northern Constabulary.
He was reported to the procurator-fiscal but the case was later dropped.
One of the complaints made to the force was an allegation that officers paid "insufficient attention" to his disabilities during questioning.
In his report published today John McNeill, the Police Complaints Commissioner for Scotland, said the response to this complaint was "reasonable".
And he called on Northern Constabulary to apologise to the man for the way it handled four of the other complaints that he had made.
These included an allegation that a community police officer, who the man claimed would know him, was not informed of his arrest "as soon as possible", the report said.
The report said: "This complaint does not appear to have been recorded nor did the applicant receive a response to it. Given that no response was issued by Northern Constabulary, the commissioner considers that this complaint was not handled in a reasonable manner."
The commissioner said he would not make a ruling on the handling of one of the complaints because the allegation and complaint was outside his statutory remit.
The report said the force had complied with its own guidelines for dealing with vulnerable people in custody.
Mr McNeill recommended that the force check its current procedures were up to date. New guidance was published by the Scottish Government in 2007.
The report looked into the handling of the complaints, not the complaints themselves.