One of my characters is a siren...

I’m not quite sure how to write him though… Any pointers?

For one thing, don’t make him sing entirely about sex. Sirens usually look beautiful, but they sing of whatever their target desires most. Food, home, family, a place to go to the bathroom, whatever fits the situation. Also, Sirens are very fishy, so include some of those qualities as well. You could give him a “head light” like an anglerfish to aid in the hallucinations, too!

also, maybe have his appearance change according to who's looking at him, for example if a lesbian woman is looking at him she'll see her idea of a gorgeous girl