Nothing good can come from his speaking with the detective, especially if he is going to be hooked up to a polygraph.
Polygraph examinations are inadmissible as evidence in criminal prosecutions, but police officers frequently use them as tools to force confessions and admissions (which are admissible in court) out of suspects. I am not familiar with detectives hooking a non-suspect up to a polygraph. Be very cautious.
The use of polygraphs has been found to be unreliable in many respects. Not sure why they would be giving polygraph to a possible witness. I'd be reluctant to allow him to participate in the polygraph as they many be seeking answers to questions other than what it may appear on its face. You very well may want to speak with counsel before engaging in further contact with the detective.
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Usually the scheduling of a polygraph means that a person is "of interest" and a potential suspect in an investigation. Do not attend the polygraph alone. Hire counsel immediately to provide a buffer between law enforcement and your son. They will press and say lots of things to the unrepresented to lull them into thinking that there are not a suspect. Don't fall for it. Lawyer up.
The detective may believe your son has information that could be of use to the murder investigation but may want to determine your son's credibility before relying upon his statements. Nonetheless, your son could also be a suspect, or his statements could implicate him in part or all of the crime without him realizing it. Your son cannot be required to submit to the polygraph, but it would be best for him to consult with an attorney before talking with the detective any further. His attorney can act as a "go between" with the detective to determine if your son is considered a possible suspect or just a witness.