I strongly believe he would have bragged about it by now if he did.
No way Elon could keep his mouth shut for that long with a secret this big.
Why would he have spent years raising capital for tesla, taking the risk of having the whole thing shut down by short-termism or nearly fold for lack of cash, when he could just cash in few bn in bitcoin?
He'll probably wait til he makes it to mars, proclaims the planet as his and starts civilization 3.0. Bitcoin then becomes the only form of planetary commerce.