Just because you think he should be over it doesn't mean he is. If he doesn't want to go, I don't think anyone should pressure him to do so.  

I agree with you, it's been 10 years, it's his kid. He should be over it enough to go to this open house. What's he going to do at graduatioin? Will he be over it by then? When his child gets married? Going to something for your child when someone is there you don't like don't mean you condone with happened. It just means you love your child more than you hate the other person. 

Is BM hosting the open house? If so, I can see not going, especially if it's in her home/the marital home. But, your SO needs to consider doing something for SS - like taking him to a nice dinner or hosting a cook out with SO's family - if he isn't going to participate. SS shouldn't be "punished" by his dad over the sins of his mother.

Now, if your SO were going to skip graduation, that's where he is being unreasonable. But I also think BM would be unreasonable for bringing the man who broke up her marriage and her kid's family to his graduation, especially since the man she cheated with wasn't a rando but a trust confidante of her now-XH. If he were some random dude without the history, I might think otherwise.

what kind of open house is this?  selling a home?  showing off a new home he purchased?

Honestly.. if it is in any way a smallish get together.. yeah.. no way I would want to go in his position.  It's different than a graduation where he might be able to avoid the other person because he wouldn't need to sit with them.

I think in the situation where someone has had an affair.. and esp with THESE circumstances.. I would have a hard time forgiving and would not be open to seeing that person.. chips would fall where they may with that.. but my adult child should understand the difficult position attending would put me in and wouldn't hold it against me for not going.

Umm.. my father married the woman he had an affair with 22 years ago. My mother still won't be within a mile of this woman at any time. Trust me, people can hold grudges forever, especially when it comes to betrayal.

Perhaps like myself, your SS considers BM's partner family now and didn't invite him out of spite. I wouldwou look to deeply into it. If your SO is uncomfortable he has every right to be.

I think that your SO, and his feelings should be respected, but it REALLY sucks that the SS wasnt more sensitive and is basically making his Dad eat a ch!t sandwhich. Whereby he cannot win.

BM recently put DH in a  similar position. Invited herself to his departed mothers vigil, and sat her lazy self down for hours. Her arrival caused him to leave early because he does not want to be in her trashy, sleazy, cheating, mean, narcissistic presence.

I was mad for DH because that is HIS family, HIS mother. Just because BM Toxic Troll has her little DNA hooks into him, doesnt mean she has ANY rights to his family.

Slightly different scenario here... Several years ago, my ex BFF said/did awful things to me. I will NOT "hang out" with him in a group activity. When our remaining mutual friends want to hang out with me, they make plans that do not include ex BFF. (he lost a LOT of friends over it). 

So while this is not the same kind of situation, my point is that if your SO does not want to go because the Betrayer will be there, he does NOT have to go. He can do something else for/with his kid.

Honestly, it sounds to me like DH is not over his ex.

I couldn't give two sh!ts about my exH and who or what he does/did.  He's nothing to me.  I spend zero time thinking about him, and I don't care if he's at an event or not.