The teenager's sister, Charlotte Cheng, told the CBS affiliate that her brother had symptoms of Covid-19 including nausea, vomiting, and chest pains.
“That list that pops up, a little bit, on that initial search, he had all of those symptoms," she said.
It is not yet clear what caused his death, the broadcaster reported. His parents confirmed their son's death to KABC-TV.
His mother, concerned that he may have contracted the novel respiratory disease, took him to hospital on 9 July to get tested for the virus but his family said the test came back negative, the broadcaster reported.
“His symptoms matched, but then the test came out negative,” his sister said. “So we were a little bit confused.”
The 13-year-old had to continue to self-isolate due to his ongoing symptoms and despite signs that his condition was improving throughout the week he later died in his bedroom.
“We went to go check up on him, like we normally do,” Ms Cheng said. “He wasn’t answering. We found him passed out in the room.”
The family said they have no idea where he could have hypothetically contracted the illness as they were staying vigilant even before the teenager became sick.
The results of an autopsy to determine the cause of death is reportedly expected in the coming days, KCBS-TV said.
Friends of the family have set up a GoFundMe page on their behalf to help raise money for funeral arrangements.
Nicole Weinstein, who is listed as one of the fundraisers, told KABC-TV: “The minute I heard, it was like a huge bright light had been extinguished. He had so much charisma, so much positive energy. He was class president last year."
“It's very, very shocking, saddening, paralysing,” she said.
“He had this, like, dream of wanting to go to the Olympics,” Ms Cheng told KCBS-TV.
California has seen a record spike in coronavirus cases in recent weeks amidst the states re-opening, forcing Governor Gavin Newsom to reimpose some sweeping restrictions in the state.
The state has recorded over 392,000 cases of the virus and more than 7,700 deaths as of Monday.