I am under the impression, and that is just an impression, that very few of older members here are or have been under guardianship. Some have lived in shared housing, some live in institutions, but most are legally on their own, and that appears to include those who still rely heavily on their parents. I think you would find, were you to ask in the general forum, that the distinction is that guardianship questions ones ability to make decisions, and most HFA or AS feel quite confident in their thinking processes, just not in their interactions with the world. If you have a good relationship with your son, I would lean towards not making a legal guardianship, and allowing him to choose what assistance he does or does not want from you.
It doesn't hurt to know what the options are for guardianship, should your son experience a sudden and severe regression (which can happen), and to have much of the mechanics in place so it could be enacted quickly, but I think its going to be important for your child to feel that you believe in him, and that means no guardianship.
But, heck, this is pretty far out of my personal experience, so I could be totally off base.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).