If the probation officer is not interested in finding your son, you have a problem. You don't specify the crime or crimes that lead to his incarceration in the first place. The probation officer would be more interested if the crimes were violent. If you know where your son is and have any influence over him, at all, try to contact him and try to convince him to turn himself in. Usually, if you can't get something done by the authorities after notifying them once, a second call, asking to speak to a supervisor or manager might get a better result. Other than those suggestions I don't think you need legal advice, Your question is more about parenting and your son is an adult now, by law, so he will make his own choices and there's not much you can do.
This answer is not intended to be legal advice in a lawyer/client relationship. Misunderstanding of the answer or use of the answer for any illegal purpose is not the responsibility of the writer. The answer to any question in the Avvo website is constrained by the limited content of the question, an incomplete description of the facts underlying the question or a wrongful motivation for the question.