This is a Social Security question, not a probate question. I edited the practice area and tags on the post so you might be seen by attorneys who work in the appropriate areas. However, you could just call Social Security and talk to someone there. In general, they will be the ones best able to answer this type of question.
This answer is not intended to provide you with specific legal advice regarding your situation, or to create any attorney-client relationship. The intent is only to provide general information. You should be aware that you cannot rely on this answer to provide you with any protection against tax penalties. You should always consult your own attorney in order to obtain legal advice.
What exactly are you talking about? You need to re-post your question with facts.
What kind of benefit was your son receiving and why? Why did it stop because your son is 16? What were the benefits from? Social Security from someone else? Or some kind of death benefit for the child's other parent who died? If Social Security, you need to contact your local Social Security office. If it is some other kind of benefit, you either need to contact the human resources/employee benefits department at the employer or whoever is administering the plan benefits, like an insurance company.
Your question indicates your son is getting survivor's benefits which would be the result of a death of a parent. These continue to age 18, or 19 in some cases. In any event the amount would not change. However, you then indicate that "he's doing better in school" which seems to indicate that he may have a disability. If he is getting money due to a disability, that is a completely different question.
Actual legal advice can only be provided by an attorney licensed in your jurisdiction, with experience in the area of law regarding your question. The information provided is for informational purposes only and is not legal advice.