South Wales Police said around 50 officers are deployed in the "extensive" search for Ben Thompson, including a helicopter and search dogs.
Chief Inspector Eddie Ough said he believed Ben had left the city centre to try to walk back to where he was staying that night.
He asked people in the west of Cardiff to look in their outbuildings, as "Ben may have tripped, fallen, hurt himself, be unconscious."
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Not a bad day today, with warm sunshine! Unless you live in the south, in which case you may have some sharp showers to deal with!
A generally dry night with clear spells in the north. Showers will push in to the south by the early hours!
Finley Thomas was a "lovely, happy beautiful baby" and changed in the months leading up to his death, a court has heard.