A Sydney man's odyssey to find his abducted son is about to end with a happy reunion in Amsterdam.

Ken Thompson's desire to see his son Andrew again was so strong he quit his job as a deputy fire chief, mounted a bicycle, and rode 6,500 kilometres through Europe to find him.

Andrew Thompson was three when his mother took him from Australia during a custody dispute, and is now six.

It is understood he came to the notice of authorities when he was enrolled for school and that his mother, Melinda Thompson, is in police custody.

Now Mr Thompson is in Amsterdam, nervously awaiting clearance to see his son for the first time in nearly three years.

"The Australian embassy is now talking to the authorities here with a view towards doing whatever can be done to enable me to see Andrew in a way that is not going to be too traumatic for him," he said.

From Amsterdam, he told 702 ABC Sydney's Adam Spencer that the news that they are about to be reunited is still sinking in.

"It was the day after Father's Day and I thought, is this some kind of Father's Day cruel hoax?" he said.

"It really wasn't until late this afternoon that it finally hit me as I was nearing Amsterdam on the train that this was true.

"I got off the train in Amsterdam at 5 o'clock and thought I can't believe this, I can't believe that I'm here, and I can't believe that Andrew and Melinda are somewhere in this city."