It would be important to know how he came to the U.S. (legally or not), as well as how it was possible for both parents to get permanent residence without doing so for their son too.
More information would benefit an accurate answer. Considering his parents are permanent residents and his length and age of entry, many possibilities may exist for him. Contact an immigration attorney ASAP.
He should consult an experienced immigration attorney and should have that attorney examine any and all immigration papers that he may have.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.