Fletcher Porrit, was injured by the Cleveland Police dog when it was called to an unrelated incident.
He was hurt near Farleigh Close, Billingham, at about 20:00 GMT on 3 March, the Independent Police Complaints Commission said.
Commissioner Carl Gumsley said it was "vital that we collate all the relevant information".
A witness appeal has been launched after a 10-year-old boy was bitten by a police dog.