Hey, I have an interesting perspective on this.
I know that teenage boys are horny and think with their other heads and all that. I know it's fairly common for boys to do this sort of thing. But it can go too far, whether they are aware of it or not.
Around 12/13, I noticed my underwear started to disappear. I thought it was strange but figured the laundry just ate it or maybe my mom threw it away or something. I don't know, I wasn't too worried. I had 2 stepbrothers, who were basically like my brothers because I knew them since I was like 5. The older one was 16, so about the same age difference.
Shortly after this happened I started waking up sometimes with my pajama pants pulled down. At this point, I started to become worried. I began going to bed with jeans on. It still happened (I still am not even sure how, I guess I was a heavy sleeper, though some have suggest I just blocked it out). Then jeans and a belt, and I remember waking up and bursting into tears because I didn't know what was happening. I was so young and scared.
Eventually I woke up during it happening. Yeah, my stepbrother put his hands down my pants and touched me and rubbed his cock on my pubic area and even stuck his penis into my open mouth, touching my tongue. I was awake but pretended to be asleep and was practically frozen with fear, my nervous system response wasn't fight or flight but rather 'freeze'.
This went on for a while. My stepbrother wasn't a bad kid and there weren't any signs-- no behavioral problems, he was intelligent and pretty damn normal.
But this, it ruined my life for a long time. I developed depression and anxiety and an eating disorder that landed me in the hospital a few times. I became a full fledged alcoholic by 18. I failed college twice and lost a full scholarship. I'm 23 now and just getting my life back on track-- I felt like a totally worthless human being (well, not totally true-- I felt my only worth was sexual) and hated myself.
In not saying this is going to happen, by any means. Just a cautionary tale I guess, because even 'normal' kids can get carried away without understanding the implications of their actions, and it can ruin your daughters life. Even something as simple as finding out where her panties are going can cause some issues.
Just keep an eye on your son, ok. I'm sure he's not a bad kid, but stuff can happen.