I would like to know if there are any articles on this subject. A few years ago when going through a Custody Evaluation I tried to find anything relating specifically to co-sleeping with a special needs child (autism) in my household.

My therapist had advised me at the time to not mention co-sleeping and so I relayed I slept on the couch and now that has been true since not long after that I got a pull out couch sleeper. But my son comes out to the couch every night and sleeps there with me.

He turns 9 in two days, but his level is a lot lower since he has severe autism. I just wonder if I should discourage this or let it continue?

I don't see any differece in co-sleeping between a special need or a "normal" child. He may need to sleep with you till an older ager, that's the only difference. Just do what he likes. In many cultures family sleep together till adulthood. We are the only crazy culture (the west) in history who came up with this strange idea of separate sleeping.
If you are still not sure, you can consult with Dr. Aldort by phone. AuthenticParent.com has a link to phone sessions.