S.J.
Hi T.,
I'm going to make a suggestion, I could be as wrong as I could be right here. It sounds like your son may be quite smart. You've already ruled out ADHD. It sounds to me like he knows exactly what to do to get away with what he wants and exactly how to get your attention. I would suggest you let him talk to the psychologist, maybe even go as a family. If you do, you may be able to rule our psychological causes or you may get insight on how to deal with him.
If there is truely nothing wrong with him, you and your husband are driving yourselves crazy catering to your son so he won't cry or pitch a fit. Smart boy. He knows that if he gets started, you'll give in. You may have to let him cry and pitch. Once he gets started, you can ask him to talk about what's bothering him. If he won't talk, let him cry and pitch some more. Eventually, if crying and fit pitching don't work, he'll either talk or try something else. When my son did this, I just looked at him and said, "That won't work either." Eventually, he learned that he wasn't going to control the house because we were all in fear of his tantrums. If you view it as a tantrum, it's a lot easier to let them get through it. If he tells you he hates himself (I swear little boys must all get this one from the same manual), tell him that it's OK because you and his daddy love him enough to make up for it. (Same goes for, "I'm so stupid.") At the same time, don't allow him to hurt his sister. If he does something to her, you might let her do it back to him. I had to sit and hold (restrain) my son in a chair until he calmed down a couple of times. If he's as smart as he sounds like he may be, he'll be much happier for having learned to deal with frustrations.
Good luck!!