It looks like a layer of skin is gone. I would clean it thoroughly with water and apply triple antibiotic cream to it daily.
Try to keep the tortoise from stress so it's own immune system can heal that. Proper temps help.
Remove him from any further injury
Your assumption of another tortoise bite tells me he was in with other tortoises. Keep him alone in his own area / enclosure.
Tortoises / turtles have an amazing ability to heal if otherwise healthy.
Good luck with him.
I'd also make sure that his water stays VERY clean until he has healed.
The salt wont hurt as long as you don't use much. It will kill many bacteria,but would not be good for your turtle to ingest.
Is the tail inflamed or full of cottage cheese type material? In the picture it appears to be infected?? Have yiu applied pressure to the wound to see if anything comes out?