I can relate to you, having gone through something similar when I graduated about 20 years ago. My mom was also the one who thought it was important at the time, kind of like a coming of age thing, and while my dad wasn't all that interested, he absolutely wanted to be there, and absolutely did not tolerate my mom or her family.
So the way it worked out was that I bounced between my mom and her family, and my dad and brothers. While I felt a bit bad at some points for leaving my dad alone when my brothers went to talk to our mom as well, or that he ignored her and only spoke to my grandma, he kind of chose that for himself.
I had already made plans beforehand with my mom to celebrate a couple of days later with her family, so after the whole shebang (had no interest in partying), I went back home with my dad and brothers, had a smallish celebration and that was that; family pleasing done, and I had a relatively nice experience in the end, with little to no friction between relatives.
I'm going to preface by saying that I believe forgiving and/or forgetting is a very personal/individual process. While my last relationship didn't work despite some mutual effort, I was on the receiving end of many throwings-in-the-face for things I didn't do and actively avoided committing a lot of in-the-faceing, so that's my point of reference.
If you want to get past the situation you're describing, being open and expressing your feelings and thoughts about it is a good start, and the sooner you get it out of your system, the less you'll hold on to it. You say he's basically stated what he's going to do, and might want to complement that by talking to him about your own expectations going forward; if nothing else you explicitly put it on the table that way and you might avoid cases of "but I thought..." later on. It can help both of you (but most critically you) get some validation related to how this impacts you, what you need, and that he's understood and is going to do something to meet those needs.
Like someone said 7M ago, you also need to accept that you can only change or influence some things (mostly related to yourself), and whatever he chooses to say or do is largely out of your hands. It basically goes to moderate your expectations of what will happen; oftentimes we expect change to come in streams, while reality is a couple of drops at a time. I personally believe real forgiveness comes naturally, but it can take (a long) time.
Since you also mention it, I'd suggest thinking proactively about how you express your feelings and wants. When/if what he did comes to mind during and argument later on, is it (or part of) the actual in-the-now issue, or just a convenient power play to get the outcome you want? If it isn't part of the present, there's probably a healthier way of communicating than bringing it up again.