Hi Kathy,
It might be a violation of the ADA, but you will have to ask either a lawyer with a practice focused upon disability issues, or your local government's antidiscrimination officials.
Even if it is a violation, it seems, probable that the gentleman will have to move because the landlord may not be able to provide reasonable accommodation. That building clearly is not an appropriate place for the gentleman to reside. Again, city officials and housing authority officials may be able to help find and arrange other housing, or pressure the landlord to find another place among the landlord's properties that is handicap accessible.
I am pleased that you have discovered this unfortunate situation and are willing to advocate on his behalf. Thank you!
