First of all, :grouphug:.
Secondly, I haven't walked in your shoes, so I don't have any "been there, done that" advice. But I was wondering, when your older children are adults, if they would be able to provide respite care for your youngest, so that you could have breaks from caring for him? If they could each watch him for two weeks, then you would have four weeks to travel, or have date nights, or do whatever you wanted.