Thank you for the information and your question. I have to tell you that, based on the information that you provided, I agree with the advice you received from the local attorney. You really, at this point, should make no further statement and instead, if and when your son is charged, you can work with his attorney who may be able to muddy the waters enough at trial
(should your son plead not guilty) to create a reasonable doubt about this being your son's. That said, a court
and jury would have a hard time believing that this was not your son's, since it was in his closet. But, that is up to his attorney to try to work with.
If you continue to persist with this issue, you too can be charged and both of you could possibly be convicted. That, I am sure, is not what you want to have happen. Even if your son is charged, there is much better chance that his charge will be at least reduced to a misdemeanor
and that he would be offered a plea deal that may include pretrial diversion. That I can't say with certainty, but young people are much more likely to get a break on these charges than a parent/adult.
Ultimately you have to do what you think is right, but remember if you perjure yourself at trial, you could face additional charges.
Please let me know if you need any clarification. I would be glad to assist you further if I can.