I have trouble motivating FDH to parent A__ too... And like you, I think that FDH is just totally unaware of his son.
If I'm not home, FDH will have A__ all darn day without feeding him a proper meal.
It's easier for him to let A__ play on the computer all day than to think up stuff to do with him.

But I just do things my way, and he goes along with it because he realizes that he's sort of a slacker if left to his own devices.
For example, right now, FDH wants to watch Trailer Park Boys, and A__ is right here in the room, so I pointed out that there is too much swearing in that show for a 9yo. He changed it back to the ball game.

FDH would probably do the same thing with Meet the Teacher night as your DH did... He's the king of "Oh, and we're supposed to be at this place at 6:30" when supper is already on the stove and it drives me nuts. I think it's inconsiderate of him to spring last minute plans on me... But I think it's a Mars/Venus thing: I plan ahead, while he ignores my planning ahead - He makes last minute plans, while I get mad about it. I've found that most men are like that (with some exceptions of course)

For things like chewing with his mouth open, it's quite probable that DH doesn't notice. A__ can have his face LITERALLY in his food and be poking it into his mouth with his fingers and FDH looks confused when I remind A__ of his table manners. When I was a kid and I chewed with me mouth open, it was always my mom or my older sister who reminded me to close it - I don't recall my dad or brothers ever reminding me. Maybe that's a Mars/Venus thing too?
It puts you in a hard spot though. Either you say "DH, your son is chewing with his mouth open" or you remind SS of his table manners yourself. Option one makes it seem like you're purposefully avoiding interaction with SS and he'll probably react poorly to that. Option two is that you're doing the parenting for DH, which you seem tired of.

I guess for me, it comes down to this:
If I do the crappy stuff like wiping dirty handprints off the wall and telling A__ to use table manners, FDH gets much less frustrated with A__ and they have a better relationship.
The difference between our situations is that A__ has improved from where he was last year (which is a lot like where your SS is now) while your SS is still in a rough patch.

I know you're still frustrated and SS is still misbehaving, but it sounded like he was progressing a little... Is he still?

Sadly, I don't think it's that he has improved his behavior but a) he's been at his moms alot lately and b) I had decided that most of his behaviors are a cry for attention and it's not my attention he wants it's his Dad's so I have been deflecting alot to Dad to open his eyes and force his involement.
DH says he used to chew with his mouth open too, when I ask how he stopped he says his dad thumped him in the back of the head. Now, I'm not suggesting he hit his son but thank heavens his dad did SOMETHING so he could become the guy I enjoy eating out with!
At the height of our issues this summer the only logical step to me is counseling individal and family. At that same time DH says "There's nothing I can do" or "I don't know what to do" It blows my mind. I have known this man for ten years and never did I realize this difference. Where I am a problem solver, try try try he just shakes the head or lays down and plays dead. Only in this arena, anywhere else in life he's clear, decisive and focused. I REALLY wish we could have had such a major difference on something that didn't matter so much. If we could have just disagreed on politics that would hae been great.

"Am I wrong to be irritated he's not going to be home for dinner because he is going to "make an appearance" at SS's school?"

No, you're not wrong to *feel* irritation and you're not wrong to even *show* some of it, simply as anyone would when they've planned a nice dinner at home that unfortunately gets abruptly interrupted by an important obligation outside the home, occuring at a specific time that can't be adjusted or negotiated. Sure, that's irritating when that happens. You gotta take the stuff out of the oven, wrap it all up, then later you gotta lug it all out agin and reheat it and it never tastes quite the same when you do that. That's annoying, definitely.

The only thing that would be 'wrong' is for it to become more than a simple irritation, or for you to get overly upset on any long-term basis about it or have expectations that this sort of thing won't occasionally (or more often than that) happen when you're raising kids. His kids, your kids, any kids.

Should you have been given more advance notice? Absolutely, if it was a forseeable event, as much notice as it was possible to give. Should DH have skipped out on his son's school obligation because of your irritation? Of course not. Especially if the problem has been that he is not active enough with his son's school, teachers, etc. He may not always be a 'good daddy', as you yourself have wanted, but it sounds like he was trying to be that night. And unfortunately it became one of those myriad circumstances where there was a relatively sudden choice he had to make between his wife's needs and his son's, between dinner at home on schedule and a school function that does not happen every day and which was important, from what you're saying, to fulfilling his son's needs AND your needs for DH to be a more involved father. Hopefully this was a good step and helped break DH of whatever nervousness or avoidance he has been having regarding talking to his son's teachers, etc.

So, irritation: yes, by all means, go ahead; major upset or resentment worth holding onto for longer than 20 minutes or the time it takes to Reynolds Wrap the foodstuffs: not so much. If broken dinner arrangements happen more than once every couple of months because he keeps changing plans last-minute when more advance notice is possible, then at that point explain to him what Hot Pockets are in case the chef of the house decides to take a last-minute vacation ;)