South Yorkshire Police said Christopher, age 41, was last seen on Monday, April 25, and has not been seen or heard from since.
Officers are now urging members of the public to come forward with any information about his whereabouts.
In a statement, they said: “He is white and described as about 5ft 8ins inches tall and of slim build. He has short brown hair and was last known to be wearing a grey tracksuit, blue hoody and grey trainers.
“Officers are becoming increasingly concerned for Christopher's welfare and want to speak to anybody who has seen him.
“Have you seen him? Do you know where he might be? If you have any information which may help police enquiries, please call 101 quoting incident number 532 of 26 April.”