Your situation sounds a little familiar!! I would definitely start with a therapist who does child, adolescent, and family therapy and you could go alone first to explain things, before the therapist meets with both of you. Another thing I would recommend, start making a list of EVERYTHING that seems odd or uncharacteristic. I say this because I started off just reporting the illegal or defiant or dangerous stuff, because it seemed more critical, but now I see that the other things that I put on the back burner at the time could have helped diagnose my child more accurately, had everyone taken everything into consideration. Docs have told me that sometimes problems don't surface until age 10 or so, when adolescent hormones start kicking. Also, things like divorce, death in family, learning something tramatic, etc., can trigger issues- like depression, attachment issues, etc. If your difficult child behaved much better when you'd been home a few days with him, complains that you "don't do, give, have, enough otherwise, and you recently went through a divorce, it sounds to me like difficult child could be feeling more insecure than it appears on the surface. For some reason, some kids apparently start stealing when this happens.
Don't get me wrong- I AM NOT A DOCTOR OR THERAPIST, so these are just my opinions and suggestions. There could be other causes, I'm sure. But, you've done a great thing by seeing the red flags and looking for a place to start.
Good luck and keep posting! Others have more experienced advice than me!