I'd get the guy to re-sing it and just tell him to watch his esses.
If he's having difficulty, get him to sing off axis and a little further back.
You can try foam filters and things like that, but really it just sounds like the guy is overdoing it and no one has told him.
If re-singing isn't an option try a de esser, volume automate the exact S points, or create a single band narrow q eq and set it to about -10db to really hear what it's doing.
Now move it left to right to scan for the offending range, while the audio is playing of course.
Once you find it, set the gain by ear.
