Reading your post, I can tell this is distressing. How old is your son? You don't have to divulge personal details, but has anything transpired in his life lately that has been disruptive or upsetting? I find that at some point, most boys become difficult. My son had a rough year around 4, and he's going through another tough phase now that he's 11. It's that pre-teen 'tude. I have found that my son's mood is hugely dependent upon mine. If I'm having a cruddy day and am short, it really sets the tone for his attitude the rest of the day.
I'd highly recommend the "Your X-Year Old" series by Louise Bates Ames. You can choose the one for your son's age. The books are great at highlighting age indicative behaviors. Where your child is going to shine, and where they're going to act out at the age they are. I also really like positive discipline. While I have days where I yell, threaten, and sometimes even use punitive discipline (such as taking away privileges or objects), I strive to let my son understand natural consequences for his misbehavior.
Until you know exactly what sort of behaviors are considered normal for your son at his age you won't be able to decide if he needs any sort of professional diagnosis and help. I wish you all the best.