Nah Son of Dracula isn't in the public domain. Universal managed to renew the copyrights to it continually over the years and with a "reboot" coming you can expect it and the rest of Universal's classic monster movies to remain in copyright until after we're all dead.
Anything prior to 1964 had something like 28 years of protection and required renewal to get another 28 years.
Another copyright act extended the second term to 50-something years for a total of 75 years.
Copyright Act of 1998 basically renders copyright renewal as nothing more than an optional formality.
Anything made in 1964 or after now has a maximum copyright protection term of 95 years.
So they will loose the copyright of the old films before we die.
Any new movies are a different story.
EDIT: shit, double posted.