We have had similar problems with the online game site called nitrome. A lot of the boys at ds's school play on it and there is some sort of rivalry going on between the nitrome fans and the friv (another gaming site) fans.
We started off allowing ds half an hour of his computer time to play on nitrome. But every single time, he became aggressive and shouty when asked to come off, which isn't his normal reaction, as he has always been clear on and accepted the house rules/parameters on gaming in the past.
The games weren't bad shoot 'em ups, but were much more shooty and hyper than the games he is usually allowed on his DSi. We found out that apparently they had been allowed to go on nitrome at school if they finished all their ICT classwork early and ds was going on it weekly at school.
Because he was so obsessed with it, his inclusion teacher wanted us to use nitrome minutes as a reward for some target or other he had at school at the time. She wanted him to collect stickers on a card every time he did the required thing and then be rewarded with half an our on the game site when he'd completed the card.
I had to say no, as by then I had banned him from accessing the site at home.
We then had an instance on him being allowed to use my laptop to do some research for a homework project. I left him for ten minutes while I dealt with his little sister and when I came back in the room he was on nitrome. He was distraught, extremely disappointed with himself and became out of control and inconsolable, eventually leading to him trying to knock himself out and throttle himself. 
I'm not sure what it is about these sites, but as Beemom said, there are a lot of adverts, pop-ups and add-ons, which makes for a highly over-stimulating environment and I suspect that's got something to do with why my ds got so addicted to it, although peer pressure was definitely the original motivator.
The other thing to consider is that on some of these sites the children can contact each other and chat online - which is something we have banned ds from doing as he is particularly vulnerable to bullying.
We now have a blanket ban on online gaming sites and it took a while, but ds accepts it, but we have already had him begging with regard to yet another new site that 'all the other boys are on' so far this year.
I am very concerned about secondary school next year though, as he is not allowed a lot of games that some of his peers already play, let alone the type of games that older secondary pupils are likely to have access to.
It is hard, because developmentally/emotionally ds is a lot younger than his peers and when you add that to the fact that many of them are playing games that they are too young for (wrt the rating system) that leaves a big gap between him and his peers. All we can do is set our own boundaries according to what we feel is right for our dcs, trust that we are doing the right thing and be aware that we might need to tweak the parameters every now and then as the marketplace and social scene change.
It's worth looking into software for blocking websites if you can't trust him not to go on it when you're not looking. There are different ones, some that you have to opt into what websites they can use and others where you specifically block the sites you don't want them to go on.
Can't help any further with that though, sorry, it's dh that's the tech guy in this house.
In terms of reducing his negative reactions, could you offer him a bargaining chip? Is there a game that's perhaps not as bad that he would like and you could allow him to have as an alternative to going on this gaming site at home?