Under NYS Law parents are obligated to support their children until the age of 21, unless they are emancipated earlier. You have a few issues here:
1) Do you know what your sone will be doing when he graduates? working? military?
depending on what he does and where he lives, you may be able to vacate support based on emancipation
2) You say you are paying support for the child who lives with you. Did he move in with you after the support order or before? If it was before, I suggest you read the order, you may be paying a net support amount.