The boy suffered a broken wrist, broken tooth and bruising when he was punched and kicked during the 2014 attack.
A Police Ombudsman investigation into a complaint from the boy's mother found that the officer made some initial enquiries.
However, he then failed to progress the case in any significant way.
During this time, his mother made repeated attempts to contact the officer, with no success.
The only time that she did manage to speak to the officer, was when he returned a call after she had learned that he had failed to submit paperwork relating to the case.
When interviewed by a Police Ombudsman investigator, the officer could provide no valid reason for the delay.
He stated only that he had been busy with a number of other cases and had, on a number of occasions, been extracted from his investigative role to perform other duties.
He added that he had forgotten the password for his answer phone and had not used that number for a long time, but accepted he had received one message which he failed to return.
When asked why it had taken almost a year for him to secure consent to access the boy's medical and dental records, the officer said he had forgotten about it.
Commenting on the case, Supt Jonathan Roberts said: "We regret that on this occasion the conduct of this officer fell below that which the public, and we in the PSNI, expect.
"We accept and have implemented the ombudsman's recommendations."
A PSNI officer has been disciplined for doing "virtually nothing" for 10 months to progress an investigation into an assault on a County Down schoolboy.