You know what? While it's "sad" to a point that some children weren't invited and got their feelings hurt when they found out, that's life. Not everyone can be invited to everything and guess what else? THAT'S ACTUALLY FAIR!
I know your son feels badly, and it's sweet. He's a sensitive boy and I hope he retains that gentleness for his friends. However if he has friends that are trying to make him feel guilty for being unable to invite them due to the rule of a limited guest list, then they're not being very good friends. Those other children need to learn how to cope with not always being included.
Why? They weren't rejected. They simply weren't included and it wasn't exclusion by bullying.
Your son did try to control the situation ahead of time by asking his party goers not to say anything in order to avoid hurt feelings at school. That was sweet. After the party, a reminder was even ok. Harping on it? Not ok. The reason for it is that you can't control, and shouldn't control, the actions of other people.
The little boy that shared during the news sharing thing was excited and had really cool news to share. He was busting at the seams that he got to see this really cool movie. That boy didn't do it to hurt anyone or "disobey" (so to speak) your son.
Now the lesson is also for your son to learn to cope with not being able to control other people or his surroundings. He is SUCH a sweet boy, a kind boy, and I would bet that maybe he has felt left out of a party once or twice when he overheard other children mention a party they attended and he wished he had gone to... and he didn't want someone else to feel the way he did.
I hope he knows there aren't any bad guys in this situation. God, your boy sounds so wonderful. I'm a little teary-eyed.