Report by Merseyside correspondent Andy Bonner
Jamie, 19, who has no military experience and does not speak Ukrainian or Polish, is believed to have been enticed to fight by members of online forums and comments made by Foreign Secretary Liz Truss.
His desperate mother, who does not want to be named, says she believes her son simply emailed a site to volunteer before booking a one-way ticket, travelling on a child's passport.
She says the Call of Duty-obsessed teen completed just one year of Army Cadets when he was at school, but has now signed a contract with the Ukrainian forces for at least two years.
Now his family, from Warrington, have no idea where he has gone.
She told ITV Granada Reports: "Every day you're just waiting, I'm constantly on my laptop, on my phone, just checking.
"It's taken over my life and I've got other kids as well and I explained it to my husband, it feels like I've split myself in two.
"I am now two people, I am a mum to Jamie and a mum to my other two kids. For Jamie I am panicking constantly, constantly trying to find out where he is located, to know he's safe."
When ITV News asked the Foreign Commonwealth and Development Office (FCDO) if Brits travelling to Ukraine to fight would face prosecution, a spokesperson said: "We advise against travel to Ukraine and anyone who travels to conflict zones to engage in unlawful activity, should expect to be investigated upon their return to the UK.”
Jamie's mother says he first became interested in Ukraine a few days after Russia invaded, and was keen to find out all he could, constantly asking her questions and watching videos on YouTube.
But, it was only after he left, when she was desperate for answers that his family discovered the extent of his research.
"On his YouTube account there was a lot of news, other reports from different other news places, just people uploading videos of people already going over there, to fight and to help the refugees," she says.
"He's a typical lad, he goes into his room, he's been there watching all of these, then going to work, then coming home, watching them again, watching Zelenskyy when he's been on telly, watching the Prime Minister, watching Liz Truss when she's said she would back them going over there.
"I think that has been one of the main reasons, the fact that it's been promoted, it's showing it's a good thing to go and help them, when it's not for a lad like him."
The mother of three says there was no vetting involved when Jamie applied, and instead he was simply accepted and told where to meet others like him once he landed in Poland.
She adds: "If you have military experience it's fine to go, but not someone who hasn't got military experience, not someone who has not been vetted.
"They've not had the health checks mentally and physically, they've not had the background checks, medical records etc, he's had none of that done, so for this, for him to be able to go over there is ridiculous."
The teen, who works in his local Subway and has never left the country before, booked a flight from Manchester to Warsaw, leaving early on Saturday 5 March.
He contacted a London-based organisation, writing: "Hello, I am here to sign up to help Ukraine fight off Russia. I was told to email you to get more information."
They replied thanking him for his support and asking him to fill out a form, adding a note to say he should only book tickets if he had military/combat or medical, rescue, fire fighting or mechanical experience.
Jamie wrote back saying he had booked his £45 flight to neighbouring Poland.
When he touched down he met others before crossing the border on the morning of Monday, 7 March.
His mother says: "It's absolutely unbelievable he's been able to book and go.
"Not in a million years would I think that he would be able to travel on his own for the first time with no travel experience whatsoever, he's never been out of the country, even with us."
His uncle adds: "In the eyes of the law he is an adult, but if you look at the email he sent it says 'I want to come and fight Russia', it even has spelling mistakes - an email like that you'd hope they'd pick it up.
"The fact that in 36 hours he was able to sign up is unbelievable.
"He didn't take anything with him, he just told us, 'I've got my army cadet uniform'.
The close-knit family are now desperately trying to locate Jamie, but despite their best efforts the geo-tracking they were using last places the 19-year-old just miles from the border with Lviv.
Once he crossed into Ukraine the site stopped working.
After being uncontactable for 24 hours Jamie finally messaged his family to confirmed he has signed up.
While he says he will not be able to contact them often, he has promised to let his mother know each day that he is safe.
They say they can understand why he wanted to help, but that the teenager does not understand the repercussions of his actions.
His mother said: "He thinks he is an adult but he's not, he definitely isn't an adult. The way he thinks is immature, it's immature thinking the way he thinks, he doesn't realise the severity of something like this."
His uncle added: "I think part of it is that Jamie wants to fight, he wants to fight because he wants to do the right thing.
"Jamie has that caring side of him, but there's also that side where he doesn't fully understand the repercussions of shooting a weapon, and what that can do to a child's mind."