Read the HIPPA law. It says that a patient doesn't have to sign so a relative or someone else can access to his records and seek help. My husband did that to me and only signed when he went into the hospital the last time where he died seven weeks later. Without that signed paper, you have no legal right to try to help your son. He has a legal right to refuse help. I am guessing, based on my radiation treatments for breast cancer which burned me, that his thyroid many never function well or at all in the future, so look for him to need thyroid pills the rest of his life if he is not on them already. Based on my experiences, there is little you can do to help your son unless you want to try to go to court and get a guardianship or something. Good luck with that. Sorry to sound so cruel, but I have been in this situation and the doctors wouldn't give me access to his condition and care needs. But they sent him home for me to care for him.
One doctor told me my husband didn't have a wife. He died that year in hospice. Your son doesn't want help. You could try to get mental health treatment but he will have to want to go to treatment. Sorry, I can't do more to help. You could try sending a special food you purchase and have delivered to the house with a Merry Christmas note and see if that sparks some cooperation. He can't really take care of himself and your only option is to try to get him committed to a facility but if he chooses to walk away, he can do it. So sorry.