Leo Del Pinto, 25, died after being shot in the face and chest in the town of Pai in January 2008. His friend, Carly Reisig of British Columbia, was also hit once in the chest but survived.
Sgt. Uthai Dechawiwat, who was off duty at the time, had been charged with murder with intent and attempted murder with intent, but initially pleaded not guilty.
"We received an email from Foreign Affairs stating there was a court case on July 2," Ernie Del Pinto told The Canadian Press Wednesday.
"The email stated they deleted court due to him retracting his not guilty plea to guilty on all counts."
A spokeswoman for Foreign Affairs confirmed the department had been informed of a development in the case. The reason for the change in plea was not immediately known.
"Consular officials are aware of developments in the case of Leo Del Pinto. To protect the personal and private information of the individuals concerned, we cannot comment further on the details of this case," Beatrice Fenelon said in an email.
"Our thoughts continue to be with the family and friends of the Mr. Del Pinto."
The former officer had already been sentenced in 2011 to 25 years for murdering his pregnant wife.
Word of the guilty plea provided some solace for Del Pinto. He said his son was a "good boy" who didn't deserve to die.
"I'm not going to say all closure because my son will never come back to me," said Del Pinto. "But I'm sure we will get a little bit of closure and justice. I was asking for justice from the beginning and it looks like I'm going to receive it."
Del Pinto said he and his wife Clara will leave Calgary Aug. 15 to attend the official verdict and sentencing on Aug. 20 in Chiang Mai.
It will be the first time they have visited Thailand since his son's death. Del Pinto said they intend to visit the spot where he died to try and understand what happened.
He said it has been a terrible five years.
"It's put me through hell for five years. I'm lost without my kid. He was a good boy. He did his schooling and what we asked him to," he said.
"It was a senseless death and he saved a young girl also. It's just so frustrating."