Ed's mum - you are going to love the liberating feeling of homework done at school!
No more nagging and no more battles!
Your son may well be at an advantage when he starts Rugby as he will have had a much shorter summer holiday than his prep school peer group who can have up to 10 weeks off compared to 6 weeks. I think its great that your son reads a lot, which is quite unusual for boys. Does he like crosswords, logic problems, sudoko etc... that he would happily pick up of his own volition? At least then he would be doing a little writing. Regarding computer games, I would give him a window of time each day when he can play them, and make the gap inbetween so long that he gets so bored and starts to find things to amuse himself. I've found that if the kids have three hours and no computer games they'll start doing all sorts. He'll be too old now, but the spiderwick chronicles have books where they ask you to draw or describe elves and fairies on the paper in the book, which is such a novelty for my 9 ear old - there may be other books geared for 11+ that offer the same.
At the end of the day I suppose it comes down to your sons personality and your personality. If you both get worked up then maybe its best to just do what causes you the least stress. This probably hasn't been much help!