Humans can have a tendency to invest heavily in their online or fantasy lives/personas when their actual lives/personas in reality are lacking in quality, content, and/or activity. That would be my first guess as to his motivations, anyway.
That being said, there's not too much shame in it. Many people spend the majority of their free time escaping from reality, whether it's in an online persona, a video game, a television series, a movie, etc. Or some people just escape into their own minds, dreaming up all sorts of things that they more than likely won't see in this reality, at least not in their lifetimes.
But lastly, that's just one suggestion. People can be complicated, there are other potential reasons for the sort of behavior that he exhibits. Delusional self-righteousness, extreme paranoia, the thrill of manipulation, all sorts of reasons! But usually even those reasons have reasons, but you have to start somewhere.
Physical isolation and regression from reality aren't really matters that we can do much about, given the nature of our relationship to the one referred to as "Solaris". Any assistance provided to him would be better suited from a source of the real, physical world. And even further, assistance from someone trained in the field of psychology.
But more importantly, introspection is also a key to battling one's own demons. If he refuses to look into the depths of his own personality, the subconscious motivations driving his conscious thoughts and actions, and the origins of said motivations from his own history, then there's little that can be done from the external world.