I don't see any evidence of shell rot on this picture. Can you
post another picture of what he looks like now? Did you
take it to a vet to confirm shell rot?

If the turtle is treated quickly they generally do OK. Go online
and look up treatment for shell rot but first confirm that it
actually is. If you are keeping the water clean and he has
a dry basking, heating area they generally do not development
this.

Post a new pic of the turtle please
Linda