Sorry for the delay in making a further contribution - had a bath and early night.
It's very difficult to give practical advice because so much depends on the dynamic between you and your son and the particulars of the situation. It may not seem very helpful but I think principles are better than specific advice because each parent must find their own way of putting principles into practice with their knowledge of themselves and their child. Also, a real bollock in parenting is that what works today may not work tomorrow! That is why parenting is such a creative challenge. Yippee!
First principle is that I don't think parents should accept unacceptable behaviour. Arsey is arsey and there is no need to pretend it's okay or anything other than arsey. However, what to do about it is another matter. I don't think arsey is best challenged by being arsey back because this tends to reinforce and escalate the arseyness However, an occasional shock is not a bad thing! The rest of the time it is better to undermine it. Sometimes that means ignoring it, but that can still be done in a way that makes it clear you are ignoring it precisely because it is unacceptable. As I mentioned before, humour works pretty well too. 'Kevin' is a comic character because he is funny. This perspective is both helpful as a response to the behaviour and good for our sanity! Try not to forget that despite appearances, inside 'Kevin' the child who is soppy and loving is still there. To whatever extent is possible, disregard 'Kevin' and continue to interact with that child even if he seems absent. This is undermining of the 'Kevin' part, affirming of the nice part, and reassuring to the child as a whole.
Here is some practical advice on ways that may help avoiding having to deal with Kevin in the first place:
It was very much the case with my son that certain activities and settings were more conducive to 'Kevin' emerging than others. Playing action-based computer games for more than 30 minutes or so almost always resulted in a rather nasty child. TV had a similar, albeit more diluted effect. Whenever practical, take Kevin outside and engage him in physical activities. That almost always worked a treat with my son and usually involved much less time and effort than resisting, fighting, disciplining, etc.
My son was often really ratty and objectionable immediately after being picked up from school. I used to think this was normal decompression and little could be done about it. Then I discovered that giving him something to eat would usually transform his mood. This is a good general lesson about children. They are physical beings and their darling little minds are tied much more closely to their bodies than we credit.
The second principle is accept the child but not the behaviour. However, the third principle is that if the child absolutely insists on identifying himself with the behaviour beyond endurance then the child should be punished, while making it clear that he and the behaviour are different things and he can and is making choices in all of this. Incidentally, children have very little foresight so a distant threat is almost useless. Upping the ante doesn't help either. One one occasion anger and frustration so got the better of my wife that my son ended up with something close to a lifetime television, computer, pocket money and friends coming over ban. You will always lose in a clash of egos with Kevin because Kevin's ego is stronger than yours. That is saying something very good about you.
Although it's not really possible to choose your son's friends, there are always some boys who are gentler and less competitive than others and it is good to encourage and facilitate those friendships. That is a good short-term and long-term strategy.
Sorry if this is rather general and vague. Hope there is something of help in it.