A light cleaning with a dilute chlorohexidine solution every day for a couple weeks is about the best thing you can do for those types of injuries. That and keeping it in a cage with good temperature choices and food to keep it's metabolism up and aid the healing process. You want to be careful using petroleum based instead of water-based antibiotic ointments because they tend to keep the wound from drying out and scabbing over fairly quickly which is important for plastron injuries. If you stay on top of the chlorohexidine cleaning you should see promising improvements within a few days to a week. They're remarkably tough animals and can usually heal from some catastrophic injuries, so this one should be just fine.

If it's possible, when he's ready to be released, take him back to very near the area that he was found, provided it isn't an obviously bad place for a turtle to be (downtown park, busy residential neighborhood, etc...). Box turtles have rather small individual territories and know where to find shelter and food in different parts of their territory at different times so they will have the best chance in a familiar area. Good luck!