I know I need to call the police, but I'm concerned about causing additional emotional damage. How can I handle this?
You must report this molestor in order to stop him from preying on any other kids. Ask the therapist for suggestions regarding your son's talking with the police and how you should proceed with reporting this crime. Your son may beg you not to do anything about this, but you must take action.
Your son is feeling very guilty right now because I'm sure that he believes a 13-year-old never should have let this happen. He is probably blaming himself. Please assure him that it is not his fault and that you'll be there for him throughout whatever follows. Kids can and do recover from these kinds of traumatic events. It will take time, professional help, and your unflinching understanding.