I am sure alot of people dealt with homophobic grandparents which is a big personal problem for me. My grandpa is the typical stereotypical republican conservative christian who watches fox news and tried to convince to go to the anti obama movie. My grandma has her views but she is more respectful and civil about them. My grandpa cannot even talk to a gay person because I have gay relatives and at a family reunion he basically just ignored a gay relative when everyone who is mostly conservative was civil to him. He almost wore his anti gay pin but was yelled at by my uncle to take it off. Sadly I will never have a grandpa I will come to respect. My other grandpa died 40 years ago and the grandpa I have known today i can barely relate to. I would appreciate stories and feedback on how to deal with grandparents. I think fully coming out will break the barriers in my family and end all the homophobia because it ns not like my gay relative was disowned.
If assuming your grandparents are reasonably educated and open-minded, they will be understanding and sympathetic to you if you choose to come out. My nan is 83 and credit to her, she told me of her 'happiness' that I am now comfortable in my own skin. Plus they would not want to offend your parents by disrespecting you.
It might make him think about what he says a little more. There's no guarantee, you know him better than any of us do. You're not required to share with people, it might be enough to give him pause about his comments. It would be a really beautiful thing if he promised to accept and love/support you, but again, there's no guarantee.
My two cents--There are probably a lot of grandparents out there who will value their love for you and the relationship they have with you more than whatever religious, moral, or generational issues they have with homosexuality. But there are also those who are simply too set in their ways, too convinced of the righteousness of their position, and basically too judgmental to ever come around. I worry about that with my mother, who is the age of most EC member's grandparents. I think that for people of a certain generation who were raised to accept all types of bigotry as being normal and acceptable there are simply limits to what their minds can adapt to. Obviously there are openminded 80 year olds but they may be a smaller percentage of the whole. At that point you may have to choose your battles. Best wishes.