I'm sorry no one was able to get to your question in time. I'm going to post an answer, just in case you're still interested in an answer from a mental health professional.
I think you and your husband have to put your health and your relationship back as a priority, instead of your son being the priority. Both of you are dealing with tremendous stress
, physical illnesses which stress worsens, and stress with other family members (such as your mother's care). You care and concern for your son is normal, but at this point it's not longer healthy because it's enabling him to stay childish, and is causing you and your husband damage.
Your son may not think he needs counseling or medication. He may never admit that he needs help. But he will never change until his environment forces him to change. You may been to work with a counselor on learning tough love parenting, and setting firm boundaries and timeline to help push your son out of the nest. It sounds like with his personality he will hate you and resent you for pushing him out. But if he is able to mature, he may come to realize that it was the only choice he gave you.
If he is diagnosed with a significant illness or disability he may qualify for assistance through the state rehabilitation department for training, counseling, and school assistance. But if you continue on this same path, he will use you and your husband and your marriage up, and still learn nothing. The only way to get him to stand up and grow up is going to be firm boundaries that give him no more options to abuse your love and care.
Again, working with a therapist can help you set this up, and help you process the anger, grief, depression, confusion, and other emotions you're going to have to face to get through this.
Best wishes and sincerest apologies for your wait,