Police said he was last seen at around 1.45pm in the area on April 10.
A police spokesman said: “When Adham has gone missing previously he has been found in Kent but he also has friends in Brighton and has spoken about travelling to London.
“Adham is described as Middle Eastern, 6ft and of stocky build, with short black hair and is clean shaven. He was last seen wearing a grey zip-up hooded top, blue jeans, black trainers and a dark coloured baseball cap.”
Anyone who sees Adham or knows of his whereabouts should call Sussex Police on 101 quoting serial 1649 of 10/04. If he is in danger or in need of urgent medical attention dial 999.