I am an Indian gay guy of 15 years. I came out to my best friend and he said that there is no such thing as gay. How to convince his narrow mind?

You don't convince his narrow mind. You move on from him and leave him alone.

If your sure yourself then you have accept, don't care about others. Be independent and don't get involved with bad people and company. Don't break friendship. If you will isolate yourself then things can go pretty terrible. Work and enjoy like you used to good luck.

If he is your true friend, then he'll respect you no matter what and nothing will change. You don't need to convince anybody of your sexuality. It's your choice, not his. There is no way to act gay since there is nothing different about a gay person, just who they end up with. So, if you get into a relationship with someone of the same gender, then that's pretty convincing. But don't just go out and find someone to convince him, being gay is something you choose, it's how you are. And it's a pretty big part of who you are. So if this is something that could ruin your friendship, then maybe he wasn't meant to be your friend if he can't accept and respect you for who you are and is expecting you to hide your true self from him. But try to spend time together like you always do and act as if nothing has changed.

Sadly there are people in the world that think that being gay is a choice, not how you're born. Those people don't believe that being gay exists, because they don't believe that you were born gay. It's very hard to change their minds, and trying to do so will likely make matter worse.

If you want to remain his friend you'll have to come to an agreement with him. Tell him that you'll respect his beliefs, as long as he can respect who you are. If he can't do that, then sadly the friendship won't last.

Good luck.

Why waste time convincing him of anything? He obviously doesn't accept it, so leave it alone, drop it.