There are a number of issues you need to consider. First, an adoption does not guarantee that someone will be granted US citizenship. Also, adopting someone "just to help him get a green card" is going to be a problem for the adoption agency and for the court if that is the sole reason for the adoption. The chances of adoption and granting citizenship are dependent upon the specific facts of your case.
You have important time deadlines in immigration regarding adoptions and granting citizenship. You need to consult with an immigration and adoption attorney ASAP.
You should always consult an attorney for advice regarding your specific legal situation. This answer does not constitute legal advice and no attorney-client relationship is created. Answers to your questions do not create an attorney-client relationship.
First, Mexico is what we call a "Hague Country," meaning that it, as well as the US have signed a treaty that now governs the movement of children from one country to another for adoption purposes. The child would have to be an orphan or abandoned child to be able to enter the US for adoption purposes and gain his citizenship AND would have to have Hague process approval, meaning that an agency in Mexico would have to go through the process there that would certify that he could not be adopted by a family in Mexico. To do a legal adoption, he would have to return to Mexico and go through the Hague process. ALso, since he is 14, he is running out of time to go through a legal adoption process, which must be finalized by his 16th birthday for US immigration purposes.So, based on the facts you have laid out, no, there is no real possibility of your adopting him legally. My suggestion - seek the advise of a good immigration attorney to see if there are any channels by which he can someday adjust his status.