I am a licensed attorney, and I will be glad to try and answer your question. I hope that the following information will be helpful to you, but please write back if you have any follow-up questions or need clarification on anything after reviewing the following information.
In Pennsylvania, a 303 Hearing refers to the applicable section of the Mental Health Procedures Act. More specifically, the nomenclature comes from § 303 of the Act governing an application for extended treatment. In other words, the initial evaluation occurs pursuant to § 302 of the Act, and now the law affords your son certain due process rights before that period of time can be extended.
One of these rights is to counsel, which is very important in these cases. If you cannot retain an attorney prior to the hearing – especially with such little notice – then when you attend the hearing you or your son should insist on legal counsel at that time. These proceedings are generally more informal than typical courtroom matters, although there are certainly very important rights at stake here.
Prior to attending the hearing, I would suggest reviewing the statute by the link I provided. If possible, it would be a good idea to print out a copy and take it with you to the hearing. Generally, the professionals involved in these mental health proceedings are sensitive to the needs of clients and their families. This is different from an adversarial matter in court, and if you have any questions you should not hesitate to ask them when you attend the hearing.