A: I agree that this is a serious betrayal of trust, but the important issue is whether or not your son is remorseful. If he feels bad about what he’s done, good. If, on the other hand, he’s defending himself or attempting to minimize the problem, then you have another problem on top of the betrayal.
Since you didn’t mention reactions of that sort, I’m going to give your son the benefit of doubt and posit that this is an example of an otherwise good kid doing what otherwise good teen boys sometimes do - to wit, they conduct little experiments with being bad. Most of the experiments in question - and again, I’m talking about otherwise good kids - do not indicate that the child in question is about to go off the proverbial deep end. In other words, a good kid can do something really bad and still be a good kid who grows up to be a good person.