As I've tried to explain in previous posts, some kind of intervention needs to be put in place in order to avoid harm not only to others, but to the teenage boy concerned in the original post.
Yes, I agree that society is letting him down.
I've tried not to give too much detail, but I've had the experience of trying to find some form of intervention for someone who was of very low intellect. (There was no specific diagnosis at the time. With hindsight, he had some form of learning disability rather than difficulty but perhaps a personality disorder as well.)
There was none and the outcome was unspeakably awful and tragic for all concerned.
It may be that a police report will not help in any way, but if there's even a faint chance that it would cause the authorities to seek a psychiatric report, then it's a chance worth taking.
As I've already stated, the failure to take heed of the warning signs with regard to the case that I'm mentioning meant that the person concerned will never be allowed out into society again. He is apparently incapable of understanding the impact of his actions on others and cannot now be trusted to be in contact with females of any age.
I keep thinking "If only."
No one is saying that all men with intellectual disabilities need to be locked up. However, as soon as there is a transgression of the kind described in the OP, there needs to be some kind of an investigation. If it's not at least reported, then if - God forbid - it happens again, it will likely be dismissed as a 'first offence'.