The timing of this request, since he is now 15 years may make this effort extremely difficult. If properly legally adopted for immigration purposes, he can never petition his parents for a green card as a matter of law.
You cannot petition him unless you properly adopt him as a matter of immigration law before his 16th birthday. In addition, the legal requirements of two years of physical custody can complicate matters. If he plans to come into the U.S. with immigrant intent, then he should be disqualified from getting a visitor visa.
In addition, he will need an F-1 student visa if you want him to lawfully attend a public high school for a maximum period of one year.
He may have other options that will allow him to study in the U.S., but if he intends to immigrate, then the consular officials have the duty to deny a non-immigrant student and visitor visa. Again, he cannot have immigrant intent where he wants a non-immigrant visa. The consular officials are allowed to presume immigrant intent as a matter of law, and deny a non-immigrant visa where they are skeptical of an applicant's intentions.
This is general information, not legal advice, and does not create an attorney client relationship.
I agree with attorney Dixler. You have very little time with in which to act.
You need to retain an experienced immigration lawyer immediately to review all the facts and advise you accordingly.
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.