It reads like he's accepted the caution - that means the reason the police are taking what she said as true is because your son has accepted guilt for it.
15 year olds can sexually abuse others, it's a growing issue in children with some arguing it being connected with media, particularly online, and cultural shifts.
She said yes there was no forcement
I'm not sure if you're going based just on what your son has said or what the police have said to be so certain for something you weren't present for, but either way - submission is not the same as consent (she may have felt forced to say yes), and as she is 15, then she can't legally consent anyways. So yes, it was abuse, by law.
If he thinks she is falsely reporting, then he should have been advised on that. As it is, he's accepted guilt and focus should be on what he's going to differently going forward. It may be a shock for you, but blaming it on the girl as it sounds like you are isn't going to help anything.