There are definitely 2 seperate issues
Your DH, who didn’t consider you on the day. And seems to accept you’re the default parent for stuff like this, so he doesn’t have to get involved. He sounds inconsiderate and I’d be upset at that. He should have thought you may have wanted to go along, or do something as a family. He shouldn’t have to be reminded to be thoughtful.
Your DS, who probably doesn’t really see what you’ve done other than be his Mum. He’s used to you being there and stepping up. It’s normal to him, not particularly special (to him) because this is what you do, what you’ve always done & been there for him. Doesn’t mean he doesn’t see it or appreciate it, it’s just he sees it as normal, having a mum who does amazing things for him.
Should he have considered you?, maybe, but teens don’t always think of the bigger picture.
With the excitement of the results, no doubt countless messages from friends he probably didn’t think about anything other than he’s passed, he’s off to uni and replying to his mates.
If it were me I’d arrange to take him out for dinner, congratulate him, talk about his plans, and just be proud of him, and yourself.
Sounds like you’re a great Mum. He knows this, and I would absolutely say he will one day realise all you did for him, even if he doesn’t quite think about it now. It definitely doesn’t mean he doesn’t appreciate it.