The general answer is that yes, he can. Your son cannot be required to answer questions which would incriminate him and subject him to prosecution. So if he's asked about his involvement in a drug deal, he would be able to take the 5th as to that question. It would be best that he have a lawyer, however and go over when he should take the 5th with that attorney, who could be there to protect his rights.
If he's a prosecution witness and the prosecutor wants him to testify badly enough, they can grant your son immunity from prosecution in exchange for his testimony. If he could get something like that, he would be required to answer rather than take the fifth, but his testimony couldn't be used against him.
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