I read the OP and thought that for me I would feel out of sight out of mind especially as everyone else had heard from their partners and you were the only one who hadn't. Especially when you said that you usually do some texting during the day normally and hadn't heard anything from him.
Then I read the rest of the thread and TBH wasn't overly surprised. It seems that going away for the weekend he suddenly thought he could have a weekend of "freedom" to do what he wanted, and did.
But tbh given he didn't get in contact until last night and hasn't bothered to offer up any kind of excuSe/apology/explanation I would be wondering whether this weekend has in fact made him reconsider the relationship because he doesn't actually believe he's done anything wrong or that he has to give an explanation.
Given his response I wouldn't be expecting justifications tbh but I would be expecting him to come back and say that he feels trapped and wants out. So what I would do is get in there first with what you want from the future. And I would act as if he has ended the relationship, given his response. So when he calls I would say "well, given you knew I had seen your actions online, and given you knew that everyone else knows I have seen your actions and you haven't even so much as tried to offer an explanation about it, that you assume the relationship is over and that this is what you want. Therefore we need to talk about maintenance/access etc re DD. Don't let him get in there first. He needs to see what impact his actions have had.
It's one thing to behave like a total arsehole and be caught on camera, to think "omg shit need to think up some excuse" and then ring frantically with apologies/justifications. That creates at least the illusion that he knows he's done wrong and wants things to work out. But dry humping some girl in a night club, live on camera and not even bothering to contact your partner with an explanation, not even a text when calls are missed sends out the message that he doesn't believe he was doing anything wrong - in fact that he wasn't even considering himself in a relationship at all. And that's how I would play it.
And like others on this thread, I've never got the attraction for having these stag weekends where everyone goes off and gets blind drunk for a weekend and potentially shags or half shags someone or behaves single for the weekend. At 35 he should have grown up some by now.