Several people stopped to help the 24-year-old victim as he lay in the street outside McDonald’s, in the early hours of Saturday 11 June 2011.
Police are urging all witnesses to the incident to contact them with information.
PC Kelly McTighe, of York police, said: “The victim suffered concussion in the incident but fortunately he escaped any serious injuries.
“It is believed that a number of people stopped to help him and it is an important part of our investigation that those people come forward and speak to us.
“They may have seen the assault take place and could well be able to help us identify who was responsible.
“Anyone with any information at all about this incident can pass information either directly to the police or call Crimestoppers anonymously.”
Anyone who can assist the police with the investigation is urged to contact PC Kelly McTighe, of York police, on 0845 60 60 24 7, quoting reference number 12110095708.