I’m sorry this happened and that your son and family are facing criminal consequences as well as the expulsion. I have experience both in elite school admissions with a disciplinary record (although 10 years ago now) as well as issues involving prescription medication abuse (totally separate matters in my family).
I, unfortunately, agree that this was almost certainly not your son’s first encounter with drugs and it could be the sign of a serious problem. The abuse of prescription pain meds is rampant and very, very serious. The users will lie, lie, lie and deny, deny, deny. Very high functioning students and professional workers have fallen under this bus and it is very tragic.
That said, if the criminal case resolves somewhat favorable- i.e. probation, deferred adjudication or a dismissal, you are in must better position. The fact that you withdrew him instead of taking the expulsion doesn’t get you too much, since I think the question is phrased along the lines of whether you had to involuntarily withdraw.
It can be explained with an essay about the experimentation and lapse of judgment that disappointed everyone, etc, etc. There are many more applicants with checkered pasts than those on this forum might realize. I was told by several highly selective colleges that an alcohol violation is almost expected, but a drug or academic dishonesty violation can be a deal-breaker.
