Jacob Smith suffered injuries to the side of his face, neck, right arm and back, according to his mother, Amber Smith.
"The whole side of his face, down his neck, all the way down his right arm and across his back," Amber Smith said.
Police allege the incident occurred Saturday night at the Yuba City McDonald's where both men worked. Authorities say the coworker threw the liquid before leaving the restaurant.
Amber Smith said her son had been preparing to end his shift when the incident happened.
"He was in the office getting ready to count the money when he saw out of the corner of his eye something, and he just turned and the oil was just thrown on him," she said.
MORE: Clovis woman sues McDonald's over hot coffee burns
Smith remains in the UC Davis ICU burn unit nearly a week later, undergoing specialized treatment. His family believes the substance used was hot oil.
"The pain he's experiencing is so excruciating that they can't give him a level of pain medicine outside of the ICU," Amber Smith said.
Doctors are working to reduce the severity of his injuries and limit the need for skin grafts, she said.
"They're just telling me that their goal right now is to decrease the size of the burn on his back and the neck area, to try to minimize the amount of skin grafting they're looking at having to do, but it could potentially become more painful," she said.
Yuba City police have identified the suspect as Jalani Bluett. Authorities said Bluett was arrested by Sutter County deputies and faces a charge of battery causing serious bodily injury. Investigators have not said what led up to the alleged assault.
MORE: Downtown Oakland McDonald's employees strike after store announces closure
Smith's mother said she hopes for accountability.
"I want justice to be served. I want him to serve the time he needs to serve for what he's done to my son. I mean, I don't wish bad things towards him. I just want him to understand what he did, really understand what he did, and the pain that he caused to my son is excruciating," she said.
Smith is expected to undergo surgery next week. His family said they do not know how long his recovery will take and are taking the process day by day.
Local media outlets have reached out to McDonald's for comment, but had not received a response as of Friday.