Pruners, MI did not invent the term 'threenager' for nothinng, you know! they become terribly imperious around this time.
I have just returned from 3 weeks away with only-child DS and came back paranoid that he was finally living up to an only-child stereotype, (so far avoided, including Cod's experiences,) but a quick peek at MN over the last couple of days has put it back into perspective - it's personality and age, they all have traits which make you worry or drive you potty. My current fear is that DS is developing int a neurotic over-emotional drama queen. But I bet several parents of just-turned-6's will say 'mine is exactly like that!'
So...what to do about the tendencies? Being v verbal is good - encourage. Just try toset a good example re interrupting and making little rules about waiting until no one else is speaking before starting. The bossing others will sort itself out - although you could talk to him about letting people make thier own decisions, and do a little bit with him, coach himn to use questions like 'what would you like to play?' etc. tell him it's sometimes more interesting to try out someone else's game because that would be a surprise. Walk away and totally ignore him while he is screraming. Tell him calmly and plaesantly that you like to talk to people who talk nicely to you and you don't listen to screaming because your ears hurt.
It'll all come out in the wash.