He needs to be physically present in the US, living your physical and legal custody, to derive citizenship from you. And then he has to physically show up to apply for a US passport at a post office or passport agency ... except if he left the US after deriving citizenship but without a passport*, he could apply for the passport at a US consulate overseas.
*US citizens are not supposed to leave the US without a passport or WHTI-compliant document, but I have never heard of that rule being enforced, so long as they present the passport or other acceptable travel document when reentering the US.