After an initial appeal for information on January 16, Thames Valley Police said three days later that officers believe he is no longer in possession of the rucksack.
A number of search operations have taken place, including with assistance from Berkshire Lowland Search and Rescue Team.
Detective Inspector Ben Henley of Thames Valley Police said: “We now have some further images to help in the search for Alexander. The Police and his family and friends are very concerned for his welfare.

“If you saw Alexander last Saturday or think you may have seen him, please contact Thames Valley Police. If anyone in the area could check any CCTV or dash-cam footage they have and let us know of anything you find which could be related to Alexander, please share this with us.”
Mr Stern is the head of a property company and his father, Roland Stern is the founder of an office products firm.