September 8, 2016 5:05 PM EDT

A professional mixed martial arts fighter whose 15-month-old son was killed by an alleged drunk driver over the weekend described the “horrific” pain of taking his child off life support and donating his heart.

“Being a father is the most amazing thing that has ever happened to me. But I can’t … I have never felt more pain,” Marcus Kowal told People on Thursday. Nothing comes even remotely close to this. Nothing.”

The athlete and his wife pulled their baby, Liam Mikael Kowal, off life support on Sunday after doctors declared the infant brain dead. Kowal said they donated their son’s heart to help another child live and prevent another family from going through the same excruciating grief.

“This whole thing has been torture,” he told the magazine as he held back tears. “To stand there, watching these doctors testing him, checking his pupils, his reflexes … to be there, hoping that there might be some kind of movement, so that he can remain on life support and have a fighting chance was agonizing.”

“You know the damage has been done, but you’re just hoping he will move slightly—make a small twitch,” Kowal said. “But he didn’t, and we just wanted to sit in a room and cry.”

Liam was sitting inside his stroller, which was being pushed by his 15-year-old aunt, on Saturday when a 72-year-old woman plowed her SUV into them as they were walking across a street in California, police said. The driver, Donna Marie Higgins, was allegedly intoxicated and tried fleeing, the Hawthorne Police Department said. Witnesses chased after her vehicle for about a block and blocked her vehicle with their own cars until police arrived.

Authorities said Higgins was rearrested Wednesday on suspicion of vehicular manslaughter, days after the boy was removed from life support. She was initially arrested for felony hit-and-run and felony drunk driving. Kowal said he’s now “on a mission” to get the country to make drunk driving laws stricter.

Thousands of people have donated more than $126,000 to an online fundraising page set up to help pay for Liam’s funeral, which is set for Monday. “Before Liam, we didn’t realize how powerful that bond is that you have with your child. The pain is simply horrific,” Kowal said.

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