Just coming in and reading this screams to me. KPOST, what do you mean you can't afford a lawyer? Please DO NOT go messing about in the family court and custody world without an attorney. You and your husband are in a position to have your son in counseling. You are dealing with a child who is crying and begging not to go to Dad's. You FEEL that Dad is not caring for this boy properly. He committed an act (non-deliberate exposure of himself to his son) that really was, if nothing else, a very poor decision about time and place for self amusement. Shows rather poor judgment, perhaps. The serious drinking you mention is the issue, not the masturbation.
And if he is drinking to the excess that you believe him to be, and you want to ask for his custody to be reduced, with the advice of your son's therapist considered, do not waste time and risk your ability to do so again by going to court without an attorney and a viable, presentable, reasonable "change in circumstances."
Let an attorney advise you as to whether you have this before you wander into court yourself unrepresented. If you think people are snippy here, let a family court judge get hold of you and your "I feel that...." and "it might be considered...." and how his dad isn't "showing him love." Not to mention not knowing what form to use, when there isn't a form. That is not how it works.
You will not find the court sympathetic to someone who does not know how it works. If you are so upset you are crying when he has to go to Dad's house, if you think this is that big of a deal, and you want to change your son from being 50/50 with his dad, or put a stipulation in the agreement that he not consume alcohol while he has the child, hire an attorney. You would not attempt to do your son's medical treatment yourself, why would you think you can't afford to let a professional person do your legal work? As you said yourself, CPS does not give medical advice. Free legal services is somewhere you can check if you are low income. But it is very important that you do not go into this legally unrepresented. You will, as I said, not have a number of shots at this in the court system. It is your son's life you're dealing with.