A defendant has the right to know the identity of those individuals who accuse him of a crime. it is called the "Confrontation Clause" in the constitution. His lawyer, if he knows, should tell your son who is likely to testify against him. Sometimes we don't know all of the witnesses until very close to the date and time of the trial, however.
What you ask sounds so odd that I am compelled to have you clarify-- Are you saying that (1) your son's own defense lawyer knows the names of witnesses against your son (the defendant), AND (2) your son's own defense lawyer is refusing to disclose to your son (the defendant, his/her client) the names of these witnesses?!?!
If that in fact is your question, that situation is very problematic. How can your son's defense lawyer properly prepare your son's defense, including whether your son should testify, and the cross-xamination of these witnesses, UNLESS your son's defense lawyer says to your son something like "Hey, the prosecutor may call Joe Smith to the stand against you- what do know about him?"
There's an easy solution though-- any witness subpoenae are likely in the court file, which you should be able to view.
The three gentlemen before me have effectively answered the question. I write to add that if the attorney is in fact stating to your son that he knows who a witness is but will not say who it is, the attorney may have discovered what is called a "conflict of interest." Sometimes these arise when an attorney has represented someone who wants to or has to testify against the attorney's client. Since the attorney has the responsibility to protect both the defendant and the witness, he should get out of the case as soon as possible, revealing only what information is necessary to get out of the case. Your son should explore with his attorney whether or not this is what is happening.
Absolutely not. Your son's attorney has an ethical - and constitutional - obligation to share all information learned through the course of representing him. There may be a misunderstanding with the attorney, who might know of the existence of a witness, but because of the circumstances of the prosecution (e.g, confidential informant), the attorney has not been provided with that person's identity. Either way, your son should clarify what information his attorney knows so that he can prepare for trial.