Police are appealing for more information about the attack, which happened at Mavericks 80s Lounge in King Street, Huddersfield town centre, at around 2am on Saturday, March 16.
The suspect approached the victim at the top of the stairs near to the toilet and wine bar and pushed him.
The victim needed hospital treatment.
The suspect is described as being a white man, in his late twenties, approximately 6 foot tall, and at the time was wearing a red jacket and dark coloured trousers.
Any witnesses are asked to contact Kirklees CID on 101, quoting 13190138191.
Information can also be passed anonymously to the independent crime fighting charity Crime stoppers on 0800 555 111.