I would ask others on this forum what they think about the advisability of making a report.
Good morning, threw a lot. Any specific questions you might have, I would start a new thread. I would frame the question(s) very specifically in the title. There are people here with a great deal of experience and wisdom. They cannot rightfully tell you what to do, but they can tell you how they would evaluate the situation, what they see as your options, and how they might respond in your shoes.
Let me tell you what I think: your response to your son has been picture perfect. On that score, rest assured. The question in my mind that is the most loaded is this: the filing of the police report, to do so or not, and how to broach this with your son. That is what I would ask the community about.
I would think that your son could start therapy with the victim witness therapy benefits (Victim of Crime Act) immediately after the filing of the police report. Because you are the parent most likely you could participate to an extent. You would have a choice of therapists, with diverse treatment perspectives, all competent. I might google to get general information about the benefits and how they work. I might verify how the benefits work in your county by anonymously calling the district attorney's office and requesting application materials and a list of therapists.
If you do decide to go ahead with the therapy, think about expressive arts therapy which would involve art, dance or music, etc., and would have a large non-verbal component. I believe It might be as effective as "talk therapy" and easier on your son. You can google "art therapy for sexually abused boys." (Or alternatively, Dance Therapy, Expressive Art Therapy for sexually abused children.) I hope you are in an urban environment, where there should be more resources. But any USA county should have therapists that work through Victim Witness.
Finally, if finances permit, think about a martial art training for your son. I like Brazilian Jiu Jitsu or Capoiera because that is what I know-through my own son, and I am thinking about the former for myself. They are for self-defense but also have a strong cultural component that seeks to build confidence, teach honor, self-esteem, personal autonomy, self-direction, responsibility and self-control. I think such a focus would help your son to deal with his guilt and his sense that what happened was his fault. It would change his sense of himself into somebody who is strong and active, can control his environment and his responses. They teach a worldview that can be very beneficial to kids and adults too, in handling life and ourselves.
You could do it too! My son shakes his head when I talk about it, (Mom. You don't understand. It is about submission.) And refers to me as the "elderly lady" who came to visit the gym.
The "mestres" or teachers typically take a strong interest in the well-being and development of their pupils. In my community it costs about $90 a month to join such a facility. I feel fairly sure they would negotiate the price down, if need be. That is what the mestre did with my son; offered to let him clean the facility occasionally for a reduced fee. And there are sometimes free classes at YMCA's. Personally, I think this kind of experience can be as important as therapy. Of course, this activity would only make sense if your son had interest in it.
I will keep an eye on this thread and look for others from you. I am glad you have felt a bit of solace and support here.