Adam Ennis, 35, from Biggar, South Lanarkshire, has reportedly left Scotland to join 50 men from around the world to defend the streets of Kyiv.
His father Brian, who was in Thailand with his wife and daughter for three months, told BBC Scotland it was a shock to discover their son had gone to fight.
“Adam was due to pick us up at the airport,” he said.
“But his friend picked us up instead.
“His friend wasn’t going to say anything until Adam spoke to us.
“So we weren’t aware until he phoned us that evening. He was already in Ukraine at a camp.”
Adam has no military experience, but his father said he knows how to handle weapons and is “a crack shot”.
Brian said he is worried for his son’s safety, but also proud of his decision to go and support Ukraine.
“As any parents, you never want to see a loved one in any danger, and it has caused us a lot of anxious nights,” he said.
“We are worried, but he has done it for the right reasons. He hasn’t done it for glory, he is not silly. He is a level-headed person and when he got there he said he had no regrets.”
First Minister Nicola Sturgeon has said that any Scot wishing to fight should “think carefully about what we can do to support Ukraine that is actually helpful and meaningful”.
She has previously said she will not encourage people with no military experience or training to go and fight.
UK Defence Secretary Ben Wallace has also urged Britons not to travel to Ukraine to join the fighting as he said the “very dangerous” situation could lead to them being killed.
He said there were “better ways to contribute to the security of Ukraine”.