I would first tell him I was very, very sorry for him.
I'd thank him for trusting me enough to tell him.
I would not stop dating him or seeing him, I would not think less of him or ignore him or anything.
I'd be open to talk about it if he wanted to and let him know that if he never speaks of it again, I would neither.
I'd let him know his secret is safe with me.
And I'd hug him ofcourse.
This is the reaction I would give too. Anyone who would stop dating someone they cared about just because he's been through this ****, is a real **** and not worth the time of day anyhow. Same goes for laughing in his face too.