As a gay guy, I completely understand. It's really difficult in school because you're stuck with a bunch of kids with no world/life experience and with no emotional maturity. Anyone who's different is going to be singled out and that goes doubly for someone who's gay because there's so much embedded homophobia still in society that says gay people are something abnormal. It's so much better than it used to be but it's still there.
The makeup thing is tricky. Are people using homophobic language towards him because of the makeup or because they actually know him to be gay? Take away the makeup and he's still a gay kid. Is there any guarantee it'll make things better? Asking him to stop wearing it could be counter productive. He's 15. he's starting to form his identify and figure out who he is. How we presents himself outwardly is a big part of that. For him, having to hide away and repress his identify could end up doing more damage mentally than dealing with the bullying. I can't speak for him so I don't know. What I am saying is telling someone who's being bullied to stop being themselves to avoid being a target can be just as dangerous. Take away the makeup and he's still a gay kid. How much does he need to hide to fly under the radar? So many people have their adult lives plagued by repressed, internalised homophobia because of the things thy had to do to hide their identify and that can be so damaging in the long term.
Other posters are right in that it would be great to get him connected to groups where he can meet with other gay people his own age or a mix of ages. In your sons case, I don't necessarily mean support based groups that focus on working through negative experiences because it sounds like your son is overwhelmed by fears that being gay means danger lurks around every corner. Support groups could end up reinforcing that if he isn't in the right headspace. Spaces for gay teens that are focused on positivity, connection and even creativity would be great - even if you have to go somewhere a bit further afield. He can get support to process the negative experiences separately and you're already exploring those options.
I think it's also important to find more mixed spaces that he can go to outside of the school environment where he'll be welcomed and have an opportunity to see that the wider world isn't full of those sorts of people. It's fine to gravitate towards other gay people and it's important to be around people with shared experiences but it's also important to see that you can belong anywhere and that your differences don't need to set you apart. He might need to build more confidence before he gets to that stage though because there could be a few risks there.
He'll never be free from the possibility of homophobia. He'll never be able to guarantee no one's judging him for wearing makeup. What will happen is that he'll go out into the world after school, he'll find his place and his people (gay or otherwise) and he'll see that those people and experiences are the minority and his confidence will grow. Right now it's about supporting him until he gets to that stage. Get him connected to things that broaden his experiences beyond school and make sure he's supported for his mental health. He's reading horror stories and painting a bleak picture of his future so help him balance that with the positive stories that he's probably skipping over as he hones in on the things that reinforce his fears. We all do that in a lot of ways.
OP one other point (sorry this has ended up a bit of a ramble); where do you live? City? Small town? Most cities will have at least one district that's a big more wacky or artsy and tends to have people from all walks of live wandering around in all sorts of looks. Maybe take him a few day trips to places like that for a coffee and some people watching. Show him what life is like in different places and outside the confines of teen/school life.