Geraldine McAllister is desperate for news of 19-year-old Jamie, who disappeared three days ago.
He was last seen on Monday and his absence has left his family sick with worry.
Speaking through her tears, Mrs McAllister, 61, begged her grandson to get in touch and .
Making a direct plea to Jamie, the grandmother-of-10, said: "Please just get in touch, Jamie.
"If there is anything worrying you, it's not a problem. I just need you to get in touch, a phone call, a text message, anything."
Jamie, who is originally from Knightswood in Glasgow, was last seen at his flat in Rowan Road, in Cumbernauld on Monday afternoon.
He sent a text message to his friend at around 3am the same day, and that was the last time his mobile phone was used.
Mrs McAllister, a pub manager from Airdrie, told the Evening Times: "We are going through hell, it's a nightmare. I'm just numb and I can't even bring myself to think about where Jamie might be.
"We have tried everything and looked everywhere, it just feels like we're grasping at straws now.
"We don't know what the text said, but his friend said Jamie sounded vulnerable.
"That's not the Jamie we know. We're a really close family and he has never said anything was wrong.You read about this happening to other families, but you never think it's going to happen to you."
The former Knightswood Secondary pupil had recently decided he wanted to go to college to study Business Management.
His family describe him as a "polite, well-mannered, and sociable young man."
Jamie's dad Steven, who is in the Navy, last night travelled back to Scotland from Plymouth to search for his son.
"The last time I saw him was at the weekend," Mrs McAllister continued.
"Jamie didn't give any indication that anything was wrong.
"We went for dinner and then when I dropped him off at home, he said 'Bye, Gran' as normal."
Jamie is described as 5ft 9in, with a thin build and fair hair.
His uncle, 35-year-old Chris, also from Airdrie, said: "We're just living in limbo, waiting for Jamie to call.
"He's not the type of boy to get into trouble."
Police Scotland were today continuing their search for the teenager.
Jamie was supposed to go to his grandfather's house on Monday, but never arrived.
Mrs McAllister, said: "As soon as I heard, I knew something was seriously wrong. This isn't Jamie, he wouldn't put us through this."
Anyone with any information is asked to contact Cumbernauld Police Office by dialing 101.