The professor may ignore the letter but there are at least three professionals who will not- the Department chair, the Dean of Students, and the Provost. You do not need to conclude that a transfer is the right answer- professors who ignore disability services are everywhere, and there are not guarantees that it will be better at the next U.
Forward the letter with a brief timeline- when your son was diagnosed, when the letter was sent to each professor, what this professor’s response has been and why your son has been unable to get 1:1 time to explain/clarify, etc.
One of these three will likely respond within 24 hours. The administration takes this stuff seriously even if each individual faculty member does not.
Hugs to you.