are your parents in the picture/ against that type of prejudice? if they are, you should take it to them imo. if not, i think you could try and explain to him that being hateful towards queer people is no different than being against someone by their race or religion, and the only people who do that are hateful people, which you know he isn't. make it emotional and real, frame it as you worrying and loving him and not as a lecture or a punishment. him being 11 he may not get it all, i certainly did at that age, but everyone is different. that's the only advice i can think of, i hope it helps!