You can purchase a power of attorney form at any legal stationery store - make sure it is the correct form, as it changed this year. You would prepare it and your son would have to sign it in front of a notary and possibly two witnesses, depending upon the use you are proposing. You should know that certain financial institutions will not accept a poa unless it is on "their" form, even though this is of doubtful legality.
Your son needs to have the capacity to appoint an agent or attorney-in-fact under a POA. If he does not have capacity, you will need to establish a guardianship or conservatorship - depending on the laws in your state. Although I am not licensed in New York, I am sure that there is a legal aid service that you can utilize. You can contact the local bar association, the state bar association, or a legal aid department in a law school for low/no cost assistance.