My head is screaming he needs phychiatric help !

Although you can try to be supportive I dont think you have the tools to help him and with help you may just get somewhere....

Questions like why ? Springs to mind, did he think it appropriate to do so ? Did he just get an urge to do so ? This is not suggesting he is a sexual diviant in any way at all ! Just a misguided child who acts on impulse the root of this seems to be what is the impulse ?

Were it that he went out and got in a fight with someone then it would be a whole different problem, and then he would need to be treated in the same way as any other troubled child. However this act dosnt seem to be temper or anger motivated, so needs to be treated in a more subtle way.

Also from personnal experience you seem to of got to that place where there is no point to anything for your son anymore, give him back all the things you have taken away from him tell him you love him and want to help him, I dont think he is fully in control of what he is doing right now. And needs help rather than punishment (again from personnal experience).