M.P.
The part of the brain that gets gratitude isn't fully developed yet. In fact that part of the brain isn't fully developed until some time in the 20's.
I suggest you're trying too hard to please. He needs more definite boundaries. Instead of turning around for BK, tell him we are going to Chic fil a today. BK another day. And then don't respond to his tantrum.
Because he's not able to accept no, I suggest it would've been best to not start the movie. I wonder if unconsciously you're hoping that he'll compromise with 10 minutes and then another 10 minutes. You're wanting to avoid the meltdown but by trying to do that you're just postponing it. And.....I suggest because he's been able to talk you into the change he's unconsciously thinking he can keep getting what he wants. And his anger builds.
That's the most important thing about tantrums. We have to completely ignore them. They are in part a bid for attention as well as the child hoping to get what he wants as a result. Once he's calmed down then go about business as usual. Do sympathize with him and is disappointment but only after he's calm. He's not listening while he's screaming.
You can teach him an alternative to meltdowns. Ask him to use his words and model for him, "I'm angry! I wanted to watch that movie! Say the words in an intense way.
As far as teaching manners, it's good to teach children to say please, thank you etc.. Don't expect them to mean it at first. It's a manner's thing at first. As they practice saying it they do gradually come to mean it.
You can foster a measure of gratitude by modeling gratitude. Say such things as I'm glad that you did such and such. Or, I'm glad that I was able to have such and such for lunch. We have a lovely home. I'm glad for it. Casually remind them of things to be thankful for. Again, gradually, it will make sense to them but at first it's more rote than feeling.