Copyright, at least in the U.S., is automatic.

He would have to actively assert that he was releasing it into the public domain (and IIRC, although my last discussion on the topic was ~20 years ago, such a release had not yet been tested in court).

> Copyright, at least in the U.S., is automatic.

It is now, but back then a work was public domain if released without a valid copyright notice.

Charade, a 1963 film, entered the public domain immediately on release.

https://en.wikipedia.org/wiki/Charade_(1963_film)#Public-dom...

It wasn't automatic in the past, which notably led to Night of the Living Dead accidentally becoming public domain on release.

Having known of the man I would say copyright would have been, if at all, at somewhere in the bottom of bottom list of battles he was (and had to be) fighting.