These are obviously very serious charges. I believe your best course of action is to find an experienced defense lawyer immediately - also I would not post any additional information regarding your sons case. Good luck.
The potential charges against your son are extremely serious, as the impact can follow your son for many years. Your son has a right, guaranteed him under both the federal and state constitutions to the assistance of counsel. Any police officer who advised you to the contrary should be fired for cause. The threat to charge your son as an adult for exercising his constitutionally protected rights is, at the least, reprehensible. I encourage you to IMMEDIATELY speak with an experienced criminal defense attorney. There are many in your area. If necessary, use the "Find a lawyer" feature on Avvo to find one. Good luck.
I would advise retaining criminal defense counsel who has experience with both mental illness and sexually based charges. These cases can very quickly get out of hand if competent representation isn't secured quickly.
The other attorneys hit the nail on the head. Given the seriousness of this situation, and the very unique circumstances your son faces, it would be a very bad idea for you to try to do this on your own. He needs an experienced defense attorney immediately. If he cannot afford to hire an attorney, he should apply for a public defender as soon as possible. In the meantime, both you and he should not be talking to anyone about the case, including the police and prosecutors. If you try to do this on your own, the sad reality is that you are very likely to inadvertently make the situation worse for him.
This answer is provided is for informational purposes only. It is intended to provide general information and does not create an attorney-client relationship with me or my law office. Nothing here should be relied upon as legal advice, and only an attorney with knowledge of your specific case can give you advice as to how the law applies. Nothing in this response creates an attorney-client relationship and therefore these communications cannot be treated as privileged or confidential.
I agree wholeheartedly with the responses you have already received. Your background facts suggest the possibility of a defense to the charges, but such a defense will not be developed on its own or by the Commonwealth; experienced criminal defense counsel is essential without delay. Good luck.