Please don't blame him for this behavior!! Unfortunately, this is typical behavior of kids who were sexually abused. It isn't his fault. Discipline may deepen the victimization.
At this point, your only recourse is to work with his therapist. Make sure the therapist is aware of these behaviors. If outpatient therapy is not intensive enough, the therapist needs to make the call as to whether a treatment facility would be the most appropriate for him at this time until he has better control over his behavior.
If you don't feel that this therapist is effective, seek out another therapist who specializes in child sexual abuse. Please keep in mind that developing rapport and a therapeutic relationship is easier said than done with a very young child, especially when discussing very private and personal matters such as sexual abuse. This can take some time. But again, if he presents himself as a danger to himself or others, a different treatment venue may be needed.
I agree that if you don't feel the therapy is effective, look for another therapist. I have been to three therapists in my life, the first one was pleasant and gave me some good ideas and I had two sessions, the second was critical and judgmental and I had two sessions, and the third, I got a lot out of our talks and stayed in therapy with her for a long time. Even within the same profession, things are not going to be the same or equally effective with every practitioner. I am not saying your son's behavior indicates the therapist is being ineffective, but am saying there is nothing wrong with calling it, if you think it is not doing anything, and actively finding something better for him. Poor kid, none of this was his own doing, and now everyone is trying to take from him the most dramatic thing in his life. (Even when something is bad, it can still be the biggest elephant in the room, especially for a kid.) Does he have any other interests to which some of his energy can be channeled?
I'd also suggest maybe just one or two therapy appointments for you, so you can get some help with the stress of all of this on yourself.
I can tell u from experience of bieng abused in the past myself that punishment communicates to the child that the act was their fault.This causes anxiety and stress that kids dont know how else to deal with it other than to stimulate sexually.Its a trigger thats been switched on in their minds from the abuse.Punishing him for sexual behavior only enforces the behavior more and will cause him to withdraw from u out of dissapointment and failure to be able to stop on his own.My aunt made me finger her and touch her when i was six years old,i didnt know anything about sex but i knew something wasnt right about that.I would tell her i was going to tell my mom,shed lock me in the room with her when we'd go visit and manipulate me to do what she wanted.She was a teenager at the time.After that i started masturbating,by age ten i was humping stuffed animals,my pillow,all to relief myself.I never told my mom about it till two years ago,im now 24 married with two kids and that problem never led to anything else other than maturbation which i finally broke free from when i gave my life to Christ two years ago.But in that time i was always acting out when i was little and i would pkay husband wife with friends at daycare and play sex.A child at that age dosent understand why theyre doing what they are doing.To them it is normal because their abuser has convinced them it is.Any punishment against this causes anxiety,low self esteem,etc.Trust is also broken once an abuser crosses those bounds and a childs innocence is takin from them.No matter how much u love on them they will feel something missing.Im not against therapy but i dont believe its the ultimate solution.Its a wound of the heart that only christ can heal.I tried everything,but it left me angry,violent,isolated,depressed,and suicidal.Every child copes in different ways and not every outcome of abuse is the same.What worked for me may not be what your willing to try.But i know i tried everything and nothing healed me till i came to the cross.Jesus IS for everyone,but not everyone excepts Him.