Oh gosh, rozied, I'm so sorry this is happening to you. I agree, it would probably be way too much for your parents at their age. You get through to your son by setting your boundaries, practicing and communicating your boundaries to him, and following through on maintaining your boundaries. All without drama, blaming, shaming, etc. It sounds like you may have to communicate your parents' boundaries to him also. Communicate with him the same way you would communicate with any other grown adult, such as a co-worker.
Given that people with severe addiction may not be able to "get it together" enough to locate services for themselves (and therefore he is turning to YOU for assistance) you may need to do some homework FOR HIM, in order to keep him out of your hair. Get the contact information for a few places, then mail them to him before he gets out of jail.
Is there anyone in your local community you can contact such as your local social services office to get references and other information? Given that your son is just getting out of jail, I would assume he does not have employment and therefore, may qualify for benefits. A social worker there would have information on halfway houses and things like that.
I don't know if you live near Philly, but there is an incredible recovery community in Philly for folks with addiction issues. If you live near Philly, you could Google "addiction recovery Philadelphia" and see if that particular organization pops up. They have MANY types of living situations for people in recovery. PM me if you can't find it and I will try.
Take care of yourself and please try not to worry.