finally had time to read whole thread.
You mention how physical he is. My ds has always needed to let off steam physically. He often goes outside to bounce basketball or kick a football. He does football club after schoolon thurs and he is a different child. We bought him a basket ball and hoop last birthday so he had something he could do even when grass was too muddy. Getting exercise really helps. It calms him down.
If computer games are his thing then ask him to teach you one. I hate the wretched things, but spent a couple of hours last saturday learning how to play professor layton, so he could be curled up next to me on the sofa having good time together.
If his games are his thing, you could try turning the system round. No games at all unless you have earned them. You get 15 minutes every time you....
When I do this, I try to make it very accessible and not set the bar too high.
Must say though, when ds has been on the computer, he gets cross and argumentative. the computer really presses all his buttons. The worse the game the more horrible he is when he comes off, he sort of needs to decompress. So our computer times have very defined rules. On and off times. Also he is not allowed any games over his age (so not even a 12) This has had a really big impact on his behaviour.
re PIL and manners. Talk to him before you go, get him to make a plan, what can he take, what would he like to do, what boredom busters does he have? If he has a ds I would let him take it. talk to him about timing (so last weekend we had visitors to lunch including another 9yo. The deal was 1 hour with everyone being polite and eating lunch and helping clear up. then he and other ds went off to do what they wanted, with their nintendo dses)
Hand some of this back to him too. Ask him what he thinks he can do when he gets cross, make a plan together. Ask him about the baby and what he gets out of the in-the-face shout. Give him an opening so he knows it is ok to say that actually he is cross with baby. Let him know that these feelings are ok, and normal. The issue is respecting him as part of the family.