Everyone has a different reason for reading, some hunger for information, others want to escape from reality, many want to read about someone with a similar problem as theirs or a similar hope or dream. There is nothing wrong with fantasy or escapism, it does not mean that a person does not wish to live their own life, or that they wish to remove themselves from society, it has to do with broadening horizons and imagination. Think about reading about another culture for the first time, or a period long ago in history; the sense of escaping into that society, or learning what life is like for those people in that place and time. It's imagination and it's also escape. That is not to say that a person, child or no, can't end up trying to escape from their life, but it is definitely not a given.
I don't have children, but I was a voracious reader as a child, and I believe my mom wondered about the same thing you are now, Sin. My personal opinion echoes that of brolie, a child that reads a great deal can be introduced to other children that also read a great deal and thus broaden their social circle. Those children can also be encouraged to discuss their books with family, or in a journal or blog. Discussing a book can help to put it in perspective, whether you are a child or adult, and regardless of the subject of the book.
If your son has problems socializing, but loves to read, ask your local library or school librarian what events they have for children his age. Most public library have many events that bring children like that together.
Good luck!