I've found every now and again both of mine have gone through a phase like this, where they seem to change and be mean or just not be their usual loving selves. It's not very nice, but I think there are a few things you can do. The first is not to engage too much, he's not really changed, he's still the same funny loving imaginative boy who happens to be pushing a few boundaries (very upsetting when you want to have perfect nice-behaved children, but unfortunately even the most nicely brought up children can have a bit of a rebellious or disobedient phase!) So, don't transmit the message he's gone bad, transmit the message that he's still the same boy who is just behaving badly at times. It's important that you keep the faith that he's essentially loveable, even if he annoying the hell out of you.
Secondly, whoever said to step back a bit had it spot on. It sounds like he's trying to start a bit of a power struggle, 6 times out in a party is way too many and way too much attention. When he starts up, just walk away. It's all about sensible consequences. (The book by the Toddler Taming man for older children is good on this).
Finally, he's only five. I find that bright children are often intellectually clever but emotionally immature (or perhaps just normal!) Try to think about it as allowing him to develop emotionally rather than just extend his brain (which is probably happening anyway). This can mean stepping back a bit, or talking about emotions, or just understanding he is little and little children do do things like hit out or behave badly sometimes, even if we have to correct it.
It is worth talking to the school to see if there is an issue but if he's as good as gold there, then it's the normal story of taking out everything on the poor parents. It will pass, honest!