In which stage it gets distorted?
- Mic? - If here, your only option is to get him further away from the microphone or change the mic (SM7 should work)
- Preamp? - If here, just turn gain down. If gain is already all the way down, look solution from #1.
- A/D conversion? - Turn down output level of your preamp.
- Mixing? - This is the only stage any plugin can make difference. Any limiter or compressor with high ratio should do the job.
Anyway, only real solution is to give this poor guy a couple of singing lessons.