N.W.
Give him your undivided attention and a snack for 15 minutes right when he gets home from school.
This helps ease the transition. I don't know if you've ever heard 8 year olds play on the playground...but they're all pretty rude to each other.
I'm a big fan of just asking your kid (at a time he's not doing it) what you can do to help solve the problem. Even if they don't come up with a solution, they'll think about it.
There's another things I do--and I NEVER explain why--as if I'm changing the subject(when he talks back or etc), I give my 8 yr old a task to complete. "oh, run this upstairs and put it in the bathroom" or "run upstairs and touch your bedroom door and then come right back" "just do it - hurry - I'll wait for you" ...
or "I need you to pick up 3 things off the floor and put them away" if they complain/whine/ask why I just add one more thing "4 things....5 things....6 things...wow, that's a lot of things ...do you really want to make it more?...ok...I need you to pick up 10 things and put them away. ....11 things...." until they give up and put stuff away.
When they are done, if they continue being rude, I make a point of looking around to find something for them to do and then give them that job. It's always a really small job--a job that gets a tiny bit harder if they talk back....so eventually they learn that it's easier to just do it at the start. If they stop being rude, I thank them profusely for all their help and give them lots of smiles.
.....this is like a miniature time out. They either improve their attitude or my house gets really clean....and my mood improves with either outcome.