I agree with what everyone else has mentioned that 8-years old is NOT too young for CBT at all. Here is a link to a free CBT for depression workbook online:
http://s3-euw1-ap-pe-ws4-cws-docume...om/9780415399784/Verduyn_depression.pdf. There is also an app for trauma focused CBT game called Triangle of Life. While it is trauma focused, it would still give the basics of CBT.
I think the ASD diagnosis is what is likely complicating finding the root cause. If it weren't for that, I would say the symptoms do sound similar to bipolar due to the cycling (though the consistent cycling is unusual for kids, it's usually more sporatic in kiddo's), and the response to an SSRI (can be activating for those with a cyclical mood disorder). I would wonder how he would respond to a mood stabilizer. It might be helpful to also track his sleep patterns. There are also many apps for tracking symptoms of mood disorders, which may be helpful. For the ASD, applied behavior analysis (ABA) can be very helpful to regulate behaviors. But it might not be nearly as effective if there is an underlying, untreated mood disorder.
Good luck! Keep trying to find a clinician who will listen and work with you guys.
