He was a big proponent of M Theory (which is a modified version of string theory), but he's been backing off lately
The discovery that black holes don't actually exist was a big blow to him, so he's rethinking a lot of things right now. (Black holes actually emit radiation, meaning they will eventually evaporate over trillions of years, this makes them "gray holes".).
Let's not make the matter trivial. Galactic-core black holes are going to exist for what might as well be eternity. Random black holes will come and go, apparently, but do realize that these theories are not nearly as widely accepted as standard theories like gravity, evolution, and at the central piece of several theories is the most recent big bang event (debate continues regarding frequency, if it is not a single event).
M-theory and String-theory, are no where near universally accepted. Nor is Hawking-radiation, though it is generally accepted by all the prominent theorists these days it seems. It does make sense in the end, and nature generally tends toward the more mundane and simplest answer possible based on the observed details.