Emilio Dominguez was charged with injury to a child by omission and arrested on Wednesday.
On Aug. 23, authorities say Emilio Dominguez and his 13-year-old son, Eli Dominguez, took drugs together. A younger child in the home at the time reported to police seeing the father give their sibling a pill.
Speaking exclusively with ABC, Eli Dominguez's mother, Jessica Muniz, said, "He always told me when he gets older, his job was to take care of his mom, and I think that's what hurts me the most."
WATCH: Mom on son who died with fentanyl in system: 'His whole life taken'
Mom on son who died with fentanyl in system: 'His whole life taken'
"There was some initial toxicology that did show that this child who died did test positive from the initial toxicology with fentanyl in his system, benzodiazepine, and marijuana," Edward Appelbaum, an assistant district attorney in the child fatality division, said.
Appelbaum said it is tough to say if something the teen took was laced with fentanyl, or if it was known at the time to be the highly dangerous drug.
After taking the drugs, the teen started struggling to breathe.
"Someone in the house said, 'Hey, I want you to try to take care of this kid. He doesn't seem to be doing well,' and he didn't," Appelbaum said. "(The father) said to sleep it off, and the next morning, that's when he called 911, and (the son) was subsequently pronounced dead."
Appelbaum said Emilio Dominguez's lack of action led to his charge of injury to a child by omission. His charge may be upgraded as investigators learn more.
"(Eli) got his whole life taken. I feel like (Emilio) needs to have his life taken. My son was just a baby. He didn't even get to live half of his life," Muniz said.
"It's just a horrible thing," Appelbaum said. "We expect more of a parent."
A state law took effect Sept. 1 that allowed murder charges to be filed in fentanyl-related deaths.
Court records show Emilio Dominguez has served time in prison, previously for weapon and drug-related charges.
"I will never be able to see him have girlfriends, get married, go to prom," Muniz said.