DISCLAIMER: INFORMATION PROVIDED FOR EDUCATIONAL PURPOSES ONLY AND DOES NOT CONSTITUTE LEGAL ADVICE NOR DOES THIS POST CREATE AN ATTORNEY CLIENT RELATIONSHIP
A mere diagnosis of Down's Syndrome is not sufficient information to determine whether someone needs a guardianship. The typical question is whether the individual is capable of making or communicating a significant responsible decision concerning their health, safety, or welfare. If he is able to communicate and make such decisions, he can defer to you if he so chooses. The doctors or others involved in his care may be inferring that he consents to you helping him with his decisions. The down side is, you may run into a situation where someone does not believe he has the mental capacity to make significant responsible decisions and refuses to allow you to participate in a decision or refuse to accept your direction. You will then have to decide whether you need to apply for guardianship or not. There are advantages and disadvantages of being the guardian, one of the disadvantages obviously is the cost involved. One of the advantages, if you are guardian, you cannot be excluded from decisions concerning his health, safety, and welfare.
In NJ, guardians are appointed when the individual is not capable of managing his or her affairs. While you've been successful so far in making decisions for him, there may come a point in time where someone will not accept your decision-making authority (because your son is an adult). Unfortunately, if he's mentally challenged, he's not legally capable of executing a power of attorney or health care directive - thus, your avenue of recourse is to be appointed guardian for him. Most Chancery courts (where you would file the application) have a stable of attorneys who are appointed as pro bono counsel when the need arises. This doesn't say anything about your own legal costs, but a social service agency might be able to help you.
If your son is unable to manage or govern his own affairs (financial, medical, and personal affairs), it is importatnt to have someone appointed to make any important decisions that could arise. it is also helpful to have a guardian appointed to protect your son from being exlpoited financially. Applying for guardianship does come with a cost, but depending on the circumstances, could be completed fairly cost-effectively. You would be responsiblef ro your counsel fees, filing costs, and the costs of the court-appointed attorney that the Chancery Judge would appoint to represent your son during the process. You may be eligible for assistance through a legal aid program or could considre contacting one of the NJ law schools (Seton hall, Rutgers) to see if they have a clinic that handles guardianship cases. Good luck.
Please be advised that my response is for informational purposes only based on the limited inforamtion you have provided. You should not construe such it as legal advice, and should consult with an attorney for advice on your specific legal issues. My response does not create an attorney client relationship. You should not act upon my response without seeking advice from a lawyer.