Adam Young, 22, was first reported missing on March 6.
He was last seen at his home in Oxford.
But now police say he is believed to be in Southampton.
A spokesperson for the force said: “We and his family are extremely concerned for his welfare and officers have been out conducting extensive enquiries to try and locate him.”
Adam is described as white, of proportionate build, with dark hair in a short buzz-cut style.
He was last seen wearing a black hoodie with a NASA logo, dark jogging bottoms and white trainers.
It is possible he is also wearing a blue puffer style coat and/or a red beanie.
He has also been seen carrying a grey rucksack with a black and white sports bag.
Adam also has links to London.
If you have seen Adam or have any information on his whereabouts, then call 101, quoting the reference 44230100773.
Call 999 in an emergency.