The incident occurred around 1.45pm on Monday, May 2, outside the Royal Mail sorting office in Hurst Road, the police added.
Sussex Police said four teenage boys reportedly followed the victim and two teenage girls through the town centre.
On Hurst Road they pushed the victim demanding that he hands over his possessions to them, the police added.
Sussex Police said the victim did not hand over anything and was pushed again, but this time was punched in the head three times.
Any witnesses to the assault or anyone with video footage of the incident which could help officers with their investigation is asked to report it to police online or call 101 quoting serial number 708 of 02/05.
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