That said, the ADA is all about accommodating disabled persons within reason. Depending on the scope and functionality of his software, fully supporting such with a single developer on staff MIGHT be an undue burden - it all depends on the specifics of the situation and we don't know his.
Notice the usage of "Undue Burden" from the official ADA documentation below.
FROM: https://www.ada.gov/regs2010/smallbusiness/smallbusprimer201...
"The rules are also flexible for communicating effectively with customers who are blind or have low vision. For example, a restaurant can put its menu on an audio cassette or a waiter can read it to a patron. A sales clerk can find items and read their labels. In more complex transactions where a significant amount of printed information is involved, providing alternate formats will be necessary, unless doing so is an undue burden."
"It is a business's responsibility to provide a sign language, oral interpreter, or VRI service unless doing so in a particular situation would result in an undue burden, which means significant difficulty or expense. A business's overall resources determine (rather than a comparison to the fees paid by the customer needing the interpreter) what constitutes an undue burden."