Jamie Cannon was on his way to college when he went missing in North Ayrshire 125 days ago.
The 19-year-old's phone was broken and he has not been in contact with his family since - something police said was "very out of character".
Officers have spent the last four months combing his home town Saltcoats, including the golf course where he was last seen.
But with a small number of police dogs available for the case, Jamie's family have taken up the search themselves.
His cousin, Leanne Graham, told BBC Radio Scotland's Drivetime with John Beattie programme that Jamie's "devastated" parents and brothers are "praying he walks through the door".
Meanwhile they have been out every day looking for some sign of him as well as telling his story in the local newspaper, the Ardrossan Herald.
Their efforts have been bolstered a specialist search team from Northern Ireland - K9 Search and Rescue.
Leanne said: "We've actually been fundraising to support them to come over because they're a charity.
"They've got more dogs than Police Scotland - they've only got one or two which they obviously can't use every single day. Whereas the team brought over five or six and they're able to cover large areas."
Leanne said the charity have been able to search abandoned factory buildings in the area where the public are not permitted to go.
"There was one time we went in but we got asked to leave because it was dangerous," she said.
Jamie was last seen at on Thursday 20 May wearing grey jogging bottoms, grey trainers, a blue jacket and was carrying a camouflage backpack.
So far, this is what little information the family have to go on.
"There's been nothing since, nothing picked up on cameras," said Leanne. "Down at the bottom you've got Aldi, Lidl and a garage - by the time the police went to them they had all looped over, so we'll never know if he got into a car or anything.
"He's not touched his bank cards, his phone was broken and his SIM card hasn't been put into another phone and been pinged."
As well as studying brick laying in college, Jamie worked at a local Italian restaurant. Leanne described him as a "typical young man" who loved life and relaxing with his friends.
As for his mother Lynn, Leanne said she is "putting on a brave face, but she's crumbling inside" without her son. "He's not in any trouble, we just want him home," Leanne added.
Anyone with information on his whereabouts has been urged to contact Police Scotland via their non-emergency line.
Ch Insp Alan Paterson said the force remains "committed" to searching for him and are using all specialist resources available.
He said: "We are extremely grateful to the local community who continue to help us locate him. Officers have received a number of calls regarding Jamie since he was reported missing and all information received is investigated fully.
"We are in regular contact with his family and we are keeping them informed at every stage of our inquiries.
"This has been an incredibly difficult few months for them having to deal with not knowing where Jamie is and we are committed to following every line of inquiry available to us in order to find him."