Eddie,
First I am sorry to hear that you are having to deal with this. As a teacher I have always tried not to make my believes and views those of my students. I am there in the classroom to teach and help them become life long learners, function in "middle class" societal norms, and expose them to many many different ways of doing the same thing. Hunting, fishing, outdoors, archery and gathering are often in my lessons. Draped over my sofa are various types of tanned furs.
Under no circumstances should the principal categorize what he deems is "normal" for your grandson. Ethically and morally he has no right to impart his views and or values on that boy. None. This can and will fall under the First Amendment umbrella for your son. Student's are afforded "freedom of speech and expression" based on the standard of "a reasonable" person.
From this point forward document everything and ask that you/parents be present when the principal needs to speak to him. If his right to freedom of speech and freedom of expression is being suppressed you can file a Title IX complaint with the US Department of Education, Office for Civil Rights. You may also file a complaint with the campus against the principal. Doing so "guarantees" no retaliation because being picked out or singled out for filing a complaint is worse than the initial offense.
I will PM you my cell number and feel free to call me if need be. I am NOT an attorney, I am a teacher, but unfortunately as a students I was subjected to discrimination, retaliation and more discrimination. There was no recourse for me, but there is no reason for ANY body in this great nation to have to suffer such indignities.
Cipriano