Amy, I understand where you are coming from. My 18 year old has PANDAS, an autoimmune disorder that attacks the brain. He has lost 50 IQ points, developed Aspergars, Tourettes, ADHD, severe fatigue, and now this stomach issue. He will always need some kind of oversight to live independently. I'm terrified that something could happen to me and my husband. We are his court-appointed gaurdians and conservators.
It may not be fair, but we have communicated to our 2 other younger teens that they are to look after their brother when we are gone. We have gone out of our way to give the healthy children a normal childhood and plenty of individual attention. They will tell you that they never felt slighted because of their brother's health issues. Family has always come first--that's how they were raised. We expect them to live that out as adults.
Our healthy teen plans on becoming a dentist. We are saving money to put him through college so that he does not start out his life in debt. We don't believe in debt. We are solid middle class living within our means. Hopefully, our son's siblings' finances will be such that if our struggling child ever needs help after we are gone, they could help out.
Now, that said, our struggling teen wants to live on his own but within 10 minutes of us. He expects to work, but doubts he could manage full-time. He has missed 75% of high school due to illness. There is no cure for PANDAS when it has gone on for as long as it has with my son. This is his life.
We encourage independence, but realistically even the state of WV agrees that he will need oversight. Beyond helping him maintain some self-sufficiency, and asking his sibs to step in when we are gone, we are hoping to start a trust fund for him. It will grow very slowly while we put out for college, but I hope to be around for decades to fund it.
One more thought, it is very likely that your son will find a mate to go through life with. This person will be there for him when/if times get tough. The burden may not always be on you to pick up the slack.