What do you think a person's sociological imagination could be who has been homeless for 20 years?
initially being homeless because he was an addict and stole from his mom who threw him out of the house
There are many variables here. Is he still an addict? Have people helped him from time to time? Is he a complete misanthrope? Is he mentally ill? Is he in a bigger city, or in a more rural area?
Without any of that information, I would say that his experiences would include witnessing events in nature (both human and non-human) which give him much to work with imagination-wise. His survival instincts have definitely been honed if he has indeed been on the street for twenty years. He has probably learned where to easily find food and shelter, which shop owners are kind to the homeless and which ones to avoid. There may even be a "place" where the homeless gather to provide a unique society among themselves which would also be an excellent source of information. He would have stored up enough information to be able to write a book about being homeless with small vignets about the people he has met on his journey.
If he's been homeliess for twenty years, he's succeeding in survival; however, I can imagine that he would see his inability to get off the streets as a source of failure on his part and/or the forces of nature surrounding him. His drug addiction certainly wouldn't help him.