MorbidMiss is right, it really depends on the child. My 12 year old can be safely home alone for an hour or two, or even left in charge of his sister, but he stresses out if asked to that too often, and certainly cannot be asked to do that for meal or bed times. Also, I'm never more than a 5 minute drive away ... that makes a difference. I wouldn't do it if I was further. Still, some kids this age (mostly NT, I suppose) are already babysitting, so part of it is learning to let go a little. Talk to the child and see what he feels ready for.
It is a big shock to loose childcare, but more and more organizations are starting to realize that maybe Middle School kids shouldn't be left without options. Boys and Girls Clubs of America run many after school programs for Middle School kids, and some communities have community centers. If you feel comfortable with him walking from point A to point B, you can also sign him up for after school classes and activities. Another option is the children's room at the library ... many kids from my son's middle school walk there on a regular basis. Specialty game stores are another thing to look into.
If he has any kids he gets along well with, the Middle School equivalent of play dates are good. Parents are regularly calling each other to see if kids can "go home together" because they have to work. We have two families we do this with (all of us moms work part time, so it's always a hit or miss who is working when).
And, of course, there is hiring someone. Perhaps a local college student, if you have any. Just someone to be there with the child, and it's fine if they work on their own homework, etc.
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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).