Most Helpful Girl
By not going into explicit details and, to be safe, probably only hinting as to what happened.
You can give details further on, but don't do it all in one big hit, there would be so many trigger issues, and it would not be considered appropriate for a young adult story.
I could be completely wrong but in my experience, in all the books I've read, (and I've read a shit tonne) any book that goes into that kind of detail get's shifted from young adult, to mature aged readers, which is usually confusing because you can tell by the style of writing who the audience is supposed to be, and it isn't until you get to something explicit like that that you realise why it's not a YA category.
So yeah, especially because the boy is under age, allude to the fact, do not detail it.
Most Helpful Guy
Maybe if you more hint at it you may have a better chance of keeping it in "young adult" Just talk about an encounter, the fact he was drugged, and then just about him waking up and feeling ill with clothes missing. Or something along those lines.
I think as long as it's not too graphic and it's handled maturely it should be fine.