I'm not a Wisconsin lawyer, but until one does respond to your inquiry, here is some good standard advice. He has undoubtedly committed a felony. Possessing Rx drugs without a valid prescription would be a felony. Using fake ID could come under an identity theft statute. It sounds like he is addicted. He should not talk to any investigators who contact him, but should promptly see a local criminal defense attorney and get himself into a drug treatment program. Not only does it sound like he needs it, but doing so would likely help him if he does get charged with this offense.
I am a Wisconsin lawyer but it is impossible to tell from your question what charge your son would be facing. We would need to know if charges have even been formally issued first. There are probably a number of charges he could potentially face, so it will depend on what he did and the county that he did it in. Often, these cases are prosecuted differently depending on which county the offense is committed in.
If you would like to call me to discuss these issues I'd be happy to talk.
961.43(1)(a) would seem to apply to your son's situation. (From your brief facts I assume he's an adult.) That statute makes it a Class H Felony to: "acquire or obtain possession of a controlled substance by misrepresentation, fraud, forgery, deception or subterfuge." A Class H Felony carries a penalty of imprisonment of not more than 6 years, and/or a fine not to exceed $10,000. The maximum term of what Wisconsin calls Initial Incarceration cannot exceed 3 years. I don't have to tell you all the collateral consequences of being a convicted felon. So that's what "he's facing" but depending on his prior record, if any, and a host of other unknown (to me) circumstances, the final result would be just a guess. He needs treatment, and obviously a lawyer who knows what he or she is doing.