I was "sexually abused" by my cousin when I was 8/9 and he was 14/15. No one on the family knows but it did screw me up for a a long time.
The thing that I struggled the most with was that I didn't feel there was a child/victim/innocent and an adult/abuser/evil, there were just two kids that we're both a bit fucked up. My cousin had a pretty terrible upbringing, he had to be treated for heroin withdrawal when he was born and things didn't improve too much from there.
I cannot forget the look of horror on his face the first time I cried in front of him and said "please don't" - he looked shocked and said "oh my god, I'm so sorry", and nothing ever happened again. I honestly don't think he fully realised that he was hurting me until then.
When I eventually tried to seek mental health help in my late twenties, one of the barriers was being told that the police would be informed if a disclosed any child abuse. When I dared to broach the subject with a therapist, they literally stopped me talking and said before I went any further, they had to warn me about potential police involvement. I therefore didn't feel able to talk about it. I didn't want to ruin another person's life for a mistake they made as a teenager, but the threat of the police being informed meant I could never be honest.
I did eventually see a psychologist who agreed that the police didn't need to be informed if I didn't give any identifying details and was happy that no other children were at risk (which I was - I had literally spent years torturing myself going over what ifs about this, studying other girls in my family and wondering if anything ever happened to them etc - if I thought for a second that anyone else was inn danger then I would have spoken up).
So I definitely wouldn't call the police behind his back, I think it's the worst possible thing you can do. When I was in that position I already felt shame for what had happened, it made me feel even more guilty to be accused of putting others at risk by not going to the police. Your partner knows the circumstances better than you or anyone else who was there and should be allowed to make his own decision on this.
I'd also try and take his lead as much as you can and not invalidate his feelings, it might discourage him from feeling able to open up. I don't think it helps to be told that things are black and white if you really only see grey. He's had years to decide on a view of the situation which might be now be quite fixed in his head, so challenging that very abruptly could be difficult for him.
Ultimately, he's the one who was there, who knows the situation and the details, so I think his views (no matter how quietly expressed) should be listened to and respected above all else). Support him, encourage him, whatever, but please try not to make him feel wrong in some way for the way that he feels/doesn't feel.