Firstly - I think its fairly normal for pre teens to go through a rather unpleasant phase, and get a bit mouthy - they are pushing the boundaries and it is a normal part of growing up.
That doesn't mean you have to tolerate it obviously.
As far as going upstairs when he gets home from school - I think thats OK, most kids need a wind down time when they get home, I never tried to engage mine in any kind of deep conversation when they'd just got home - they are tired and need to relax. But by meal time I would expect him to be relatively sociable again.
AS far as rudeness is concerned - I don't do things for people who are rude to me. If mine were ever rude - I would walk away and say 'I will talk to you when you can be polite to me' don't get upset, don't shout back, just make it clear you do not engage when someone is shouting at you and being rude.
Also if someone expects me to do them favours or accomodate them - whether it be cooking for them,washing their clothes, giving them lifts etc, then they have to be polite or I might find myself disinclined to take them to things they want to go to or pay fares for them or whatever. If rudeness occurs in conjunction witht he computer games then you say' these games are obviously making you rude and aggressive so we will remove them until you are polite and prove that you can remain polite if you are allowed to play them'. He has so far proved that he is not old enough to cope with a 15 game, so tell him it goes away unitl he is fifteen because it has made him rude and shouty.
Being calm but firm and sticking to your guns in a non-blaming way is important - just 'if you do this then this is what will happen' in a very matter of fact way. Give him boundaries - it will make him feel more secure.
They do get past it and come out as nice people at the other end - although it sometimes feels endless at the time. Hormones are running rife at this age and have a lot to answer for !! Good luck 