The teenager, identified as Chris Singh, underwent surgery at a Fresno hospital for head trauma after he was reportedly shot by his father yesterday.
Earlier, doctors had said his injuries were life-threatening, without giving further details.
Avtar Singh, the former Army officer, who was wanted in India for the killing of a prominent Kashmiri human rights activist Jaleel Andrabi in 1996, apparently shot his entire family comprising his wife and three children, before killing himself in Selma in California.
While two children -- aged 5 and 15 -- and their mother were found dead, along with Singh, the eldest child was in a gravely injured state.
Spokeswoman of the Community Regional Medical Center, Mary Lisa Russell said, the boy was in critical condition.
Fresno County Sheriff Margaret Mims said the elder Singh called Selma police at 6:15 a m on Saturday morning (local time) and told them that he had killed his family and was now going to kill himself.
Authorities have not yet officially released the names of Singh's wife or the two sons who were killed.
According to a report in a local paper Fresnobee, Singh was arrested in February last year after his wife accused him of trying to choke her.
The people in the community where Singh and his family lived were shocked with the incidents.
"Our community is completely shocked... It's a misfortune, especially for the children who died," said Rajbir Singh Pannu, president of the town's Sikh temple. (MORE)