It relates to the NY quarantine and CA is one of the “high-risk” states. He still needs to fill out the form (he can do it online) as they may check it on arrival at JFK. The website is at traveler.health.ny.gov
One of the questions on the form is “is your final destination in NY state?” If the answer is no, it jumps straight to the end of the form and he should get a green indicator on the online form. He should just take a photo of that confirmation and show that confirmation to the checkers at the gate on arrival at JFK.