This is all very Twilight Zone, parallel universe for me. Been there. Done that. Have the scars to prove it.
One thing really stands out to me:
Quote:His behavior has really hit a high note these past 3 weeks
I think this is a sign of progress so don't give up yet. I know you're both frustrated and tired, Little Kevin (LK) is too. (Sorry if I get the names wrong - I'm not quite sure who is who.) The fact that Husband and Wife are presenting a more unified front will make LK try every trick in the book. Divide and conquer. Even if it means dividing everyone from him.
Like for you, time out caused hysterics; taking away toys didn't matter because he didn't care about "stuff" at all; curtailing activities that were more than five minutes away - ha - he couldn't think past his fury to consider it.
The whole notion of cause = effect was lost on him.
I think soz's is right with the attachment issues (she's the one who first pointed it out to me, too). It isn't ADD or ADHD but RAD. Read up on RAD (I'll try to drag out some of my old links) and talk to a psychologist again.
You'll probably hear the same thing I did - wait until he's older, but you will get some good pointers about why traditional discipline won't work and some tips on what might work.
Everyone around here is doing their morning "NOW! NOW! NOW!" but I'll look today to see if I can find some things.....