Marc Thompson said the "hardest thing is not knowing" where his twin brother Ben is.
The 34-year-old from Haverfordwest has been missing since Saturday, after he watched Wales' Six Nations game at the Millennium Stadium.
He told our Correspondent Carole Green that this would be "very out of character" for him not to contact his family.
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A new speed limit has come into force on major roads and motorways to help tackle air pollution.
A cloudy start to the week but turning drier throughout the day.
South Wales Police tweeted in the early hours of Sunday morning that they had been caught on a stretch of the M4 near Port Talbot.