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...

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.