I wish I could tell you that antibodies were directly related to diabetes or at least predicted how much your pancreas was being attacked, but it is just more complicated. Only 85-90% of T1 diabetics test positive for one or more antibodies and studies have not been able to show that antibody levels are related to the progression of diabetes. And antibody levels could simply be dropping because there are fewer and fewer islet cells left. For the most part, endos will check antibodies to confirm a diagnosis. Once you test positive for antibodies and have clear insulin deficiency problems (which your son has), there is just no further reason to test for antibodies. More typically an endo will perform a fasting c-peptide test which is more indicative of how much insulin is still being produced.
Many that are diagnosed go through a "honeymoon" in the early days after diagnosis. Their insulin production rises and falls. And it is not uncommon for insulin needs to be reduced as blood sugars normalize. So your son's reduction in insulin needs are explainable in other ways. I wish I could tell you that somehow antibodies could go away and that there are some who are cured of diabetes, but unfortunately I have not heard of any cases like that.