What if the professor demanded his students wear a specific kind of mask, like an N95? What if he required his students to take rapid tests before class? Where do you draw the line?

If an elderly person with comorbidities -- the highest risk individual in this pandemic by far -- wants to work in an environment where he will be exposed to many other people, which is inherently risky regardless of vaccination and masking (because neither offers 100% protection), he should find an employer whose rules are satisfactory to him, not impose rules that are contrary to his employer's.