I think maybe separate out your battles.
Please and thank you could well be shyness, - and I think many parents understand that shy children struggle with this. At home he should be able to manage them - as presumably he's not shy around his family. But with other adults he may struggle, Encourage him to smile if something is offered and look enthusiastic, then he may gradually gain enough confidence to verbally thank people.
Saying sorry - displaying contrite or 'sorry' behaviour would be enough for me - children struggle with the word, and it is just a word, what you really require is stopping of bad behaviour and making amends, if he can do this that is displaying an apologetic attitude. If his attitude shows he isn't sorry, then give him some time to think about it and then return and interact sensibly.
Interrupting and saying he is bored/things are boring is rude, not shy, and he is able to not do these things at 8. (He may well be emotionally immature, and not think about the effect of his words, but he's old enough to have a conversstaion about it) Explain that interrupting is rude and that everyone needs to take thier turn to speak, what he has to say is not more important than anyone else, so he needs to wait his turn (unless it is an emergency obv.) And it is rude for others to interrupt when he is speaking.
It's unkind to say things are boring or you are bored (when you are at someone elses house or attending an event someone has organized etc.) He's old enough to understand he will hurts people's feeling si f he says this kind of stuff and he needs to keep his boredom to himself. Sometimes we need to just suck up a boring event to be kind to someone. If he's bored at home - find him a job to do - he'll probably disover he suddenly has something interesting to do instead 
8 is a funny age - a sort of border when people start to expect a bit from children in terms of social awareness. But all children are different, and generally boys tend to be a little later than girls in acquiring these skills.