My five year old also started doing this recently. There is NO supervision of the other kids in our neighborhood (and we're on a very busy street, no less) and kids as young as seven are using words *I* don't even use. We finally said he wasn't allowed to play outside with them anymore. Instead we'd take him to the park down the street, but were running into the same issues. (ie talking back, refusing to come home, etc) It later escalated into not eating or doing his chores.
What we did was start taking the object in question away and putting it on a shelf in the living room. (ie toys he didnt pick up, toys he threw, etc) If he didn't eat his dinner in a reasonable amount of time (he's been taking up to two hours to eat a bowl of cereal) it got put in the fridge and he'd eat it at a later meal. If he used bad language or was being sassy, we'd wash his mouth out with soap (I know this seems a bit harsh, but he began using some VERY colorful words that are absolutely innapropriate for a child of any age and soap has nipped it in the bud).
In order to get his toys back off the shelf, not only does he have to behave for the rest of the day, but he has to do something helpful/courteous for someone else. Another thing I pointed out is that his little sister is going to learn from what he does, and if he's naughty, she will be too because she sees him doing it (but now I'm dealing with him being a tattle-tale everytime she does something).
I'd also ask his teacher if there's been behavior problems at school. Does he act like this in class? Do other kids act like this? It may explain where it's coming from, or he may just feel over-whelmed at all the changes he's gone through recently (moving, school, etc). Maybe talk a little every night at bedtime about how his day went (positive things, not about his behavior) to show that he still has some stability and control when everything is so new to him, and that you're still here for him and he doesn't have to act out to get your attention.
Good luck! I know how frustrating this is. :) Let us know how it turns out!