Don't let him define himself as 'gay'. It's a 'desire' and an unnatural one. Thats about it.

If he stops following the desire...then it will no longer exist after a while. The only reason he feels gay is because he is looking at something which makes him feel desire...so tell him that he needs to lower his gaze. After a while he will forget such a thing as 'gay' even exists.

You only feel gay if you follow your gay desires and because society keeps telling you. For millions of years people didn't feel the need to identify themselves as 'gay', because society never encouraged them and even if they did have something, they would immediately feel guilty and that was the end of it. The islamic advice for any kind of desire would be to lower your gaze and refrain from following your lusts. Then after that you won't need to call urself 'gay' anymore, cause u can only be gay if u do something of that nature, otherwise it's just a thought that crossed your mind.