I am about to finish my PhD, and I have been a TA for almost 7 years. (2masters+5PhD). I have worked with too lenient professors, too strict professors, and most troubling; I have worked with professors who would not eve bother to attend the lectures most of the time, canceling them with one reason or another and still being too strict on students because some of them have talked bad about them on social media or generally they want to prove that they are "hardcore" professors that needs to be feared and taken seriously. The only advise I can give you is; do not own the grading, do not own the class and do not own the course, do not own the students. They are not yours, but his. He can decide on whatever he wants as long as he is within his legal limits. Do it according to his wishes. In this changing of the grading scale situation, just inform him that this change might cause such problems with a level and nonemotional tone and let him decide.
You will feel much better if you do not personalize it.