Police are continuing their desperate search for a 25-year-old FIFO worker who went missing nearly five days ago, with revelations he was last seen at Trigg Beach.

William Patrick Carter was dropped outside Perth Airport’s Terminal 3 about 12.40pm on Saturday, but never boarded his flight.

About half an hour later, Mr Carter ordered a taxi to travel from Perth Airport to Trigg Beach, and was last seen near the local surf club about 2.40pm.

He has not been seen or heard from since.

Mr Carter was last seen carrying a small backpack and wearing a black t-shirt, shorts and white sneakers.

He is described as being approximately 174cm tall, of slim build, with brown hair and blue eyes.

His mother, Jenny O’Byrne, said she dropped her son at the airport after eating breakfast together in Kelmscott, where she took a selfie just an hour before he went missing.

“That photo was taken just as we left the Dome cafe in Kelmscott, I said ‘come on lets take a selfie for your sister’ because she’d asked me whether I was with him, and that was about 12.20pm on Saturday,” she told PerthNow on Tuesday.

The Bunbury nurse said she was concerned for her son’s mental health and said his disappearance was “completely out of character”.

“There are serious concerns for his outlook on life at the moment, so the police now are taking the missing persons concerns very seriously,” she said.

“He’s not well, and he’s very vulnerable and at risk and we just need a lot of people to keep their eyes out.

“This is completely out of character for him, he has been displaying some quite bizarre behaviours, he does normally take anxiety medication but he’s been off that for about the last five months and that was a scheduled tailoring off.

“But I think there’s been some other things that have gone on in his life, a situational crisis, and that’s so often the case in the mental health field.”

Ms O’Byrne described her son as quiet but widely loved, adding he had recently returned from a family holiday to visit his father in Zambia.

“He’s a very quiet affable chap, he’s someone that everybody enjoys, he’s not loud and garish, he’s very quiet, he’s been going through some challenges recently and because he’s so quiet in nature, some of those challenges have been missed because he’s not very talkative,” she said.

“As a family we’re very supportive, I spoke to him a little bit on Saturday about what his concerns were, told him he has the full support of the family.

“We’re financially very solvent here and overseas, it’s just simply a case of when someone’s not well and they’re going through a situational crisis they don’t see the world as we would see it, they see it altered.”

Ms O’Byrne urged people to check-in with loved ones, and said Christmas could be a particularly “lonely” period.

“This is really a good lesson for everyone that mental wellness is prevalent, and is just so important in everyone’s family because you just don’t know what can cause them to be unwell,” she said.

Anyone with information is urged to contact police immediately on 131 444.