My son was diagnosis with bi-polar when he was 12..........but I knew from age 2. I went through hades and back to get him the right medications, and the right care, because I just knew. doctors told me he was this or that - and put him on medications that made him worse. Finally, after he started exhibiting true manic signs, like sleeplessness, did the doctors feel like the could give him a true BiPolar (BP) diagnosis.
My suggestions would be:
*Not move to a new city. This would only escalate his stress level and chaos quotient. Especially with a potential "new dad" in the equation.
*Not spank him. This only escalates his aggression. Read The Explosive Child by Ross Greene. He gives concrete ways to deal with these kids.
*Change psychiatrists and get one that is on board with mood liable kids.
*Once psychiatrists have been changed, insist that your child get off medications that are stims, like Focalin, which can increase aggression impulsivity - and medications that are SSRIs like Zoloft, that can also increase aggression and impulsivity - and try mood stabilizing medications like Lithium, Depakote, Trileptal, etc.
*Get difficult child a full medical work up that include psychological testing.
Good luck..............we have walked in your shoes.