I'm not sure if you were actually asking for advice about your son (he's 5, right?), but here's what I do with my Kindergarteners. First, I taught them about what it means to be respectful. This is something we discuss all the time, so they have a good idea about what that word means. Second, I make sure I give them choices ALL the time. They can be two choices that are both things I want, but since they get to choose between them, they feel empowered. That empowerment goes a long way. Giving them many choices throughout the day also gives me the power to say, "You get to choose things all the time, but talking to me (or whoever) like that is not a choice." <-- if you don't use this too much, it is really effective!
I also ask them if there is a nicer way to say what they just said. If they say "Gimme that!" & I ask for a nicer way, they usually come up with "Can I please have that?" If he can't come up with a nicer way on his own, suggest one for him so he has that tool for next time.
Turn things around on him. Kids at this age are concerned about themselves, not so much others. If you put things from his perspective, ("Would you like it if I said that to you?") he is better able to see why it's not ok to be disrespectful. Make sure you have a lot of kindness in your voice though when you say that to him (even though you probably won't be in the mood for kindness, lol!).
Finally, any time you notice him being respectful on his own, point it out! Saying "I notice how kindly you were talking to your sister" or "I really like that you remembered to use nice manners" emphasize the positive, so he feels good about doing these things all on his own, and he'll want to do it more!
I know that was really long (I apologize), but I also know how overwhelming 5 year olds can be sometimes, so if you wanted the help, I hope it does help. If not, just ignore me!
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