Is it sexual assault, is it just laddish behaviour, is it bullying?
It depends on who did it, to who, when, in front of who, ages of both etc etc.
I taught in secondary education and over the years had to deal with laddish behaviour (for want of a better word.) And sometimes to pre-empt it.
We used to talk about the problems in life a conviction or warning could cause in future life.
It might not be intended as SA, it might be intended as a joke.
But...... do something like this to the wrong person with the wrong parents and you'll end up with sexual assault being mentioned at a police station.
What started as a joke soon stops being one.
You are well within your rights to report it to the school or bypass them and go to the police.
At the moment he doesn't want you to take it further - but on the other hand he did tell you. Clearly disturbed by it.
Maybe see how the start of next term goes?
Your son has definitely been a victim.