I don't think there should be a limit to thinking about this.
Better to be over prepared than under I would say.

I myself am autistic aged 28, I'm on the high functioning side of things, My problems manifest as mainly difficulties in social situation, reading intent and emotions, interaction face to face and talking to people. It's exceptionally stressful and exhausting. I rarely leave the house or interact with other people other than on the internet.
I currently live with my father who cares for me, deals with the shopping and welfare paperwork and a host of other things.

I honestly can't think about what would happen if my dad died, I'd probably try and continue as I am but I feel like I'd burn out and break down before long. Time will tell though until then I'll work on becoming more independent.

In regards to your son, you just have to do as much as you can. You are already doing a fantastic job that much I can already tell. You can try and teach him as many basic things as possible if he is able to learn and retain that information, if he's on the low functioning high needs side of the spectrum then the alternative is introducing a 3rd party carer you trust or something along those lines.

Anyways, Keep up the good work and thanks for the fantastic post. I always enjoy reading other peoples experiences with autism.