I'm genuinely curious - why is the housing complex now obliged to transfer him?
They're obliged to transfer him because it's unlawful to refuse to make reasonable accommodations that are necessary to afford a person with disabilities an equal opportunity. This part of the Fair Housing Act is at 42 U S Code 3604(f)(3)(B) at http://www.law.cornell.edu/uscode/42/3604.html. Here's another case (summary) with a $125,000 settlement for refusal to transfer a family to the ground floor: http://www.justice.gov/crt/about/hce/documents/casesummary.php#hasa. And in that case, the whole building was wheelchair accessible.
Here's how the DOJ attorney summed it up in the $1.25 million case (excerpt from link in post #6):
Thomas E. Perez, the assistant attorney general for the Justice Department’s Civil Rights Division, said in a prepared statement that the settlement should send a strong message to apartment owners and managers.
“Property owners and managers have no excuse for violating our nation’s fair housing laws by refusing to accommodate people with disabilities,” he stated. “Equal access to housing in the United States is a fundamental right, and this nation will not tolerate discrimination in housing.”
“Property owners and managers have no excuse for violating our nation’s fair housing laws by refusing to accommodate people with disabilities,” he stated. “Equal access to housing in the United States is a fundamental right, and this nation will not tolerate discrimination in housing.”