Hmmm .... well it sounds to me like he's just going through first time parent issues. Many people I know who have been first time parents feel that the first birthday is a HUGE deal, and to be fair to them, it is. Your child only turns 1 once. And when it's your first child, it's an even bigger occasion because it's not just a first for their child, but for themselves as well, and truth be told, the first birthday is more for the parents and family than it is for the child.
On the other hand, I think it's really sweet that you offered to put them up in a hotel. And I don't think you're wrong to be upset, because your wedding day is a HUGE event in your life. Maybe try to compromise. How about letting him know that you understand the following day is a very big day for him, and let him know that you don't mind if they leave early. I realize she's your MOH, and that might mess up some stuff, but you do have time to plan for it. Let him know that you realize that he's starting traditions with his baby, and that you respect that, but that you still really want him there. Let him know that it won't be the same without him there, and even if they can't stay for the whole thing, you hope that they will be able to come for part of it.
It's a really tough situation to be in. The two events are so respectively big in each of your lives, and falling one right after the other makes it tough. Your friend may be wanting a quiet evening at home with his baby before that big milestone is crossed, and I bet he's conflicted, because he probably really wants to be at your wedding as well. Are you going to the party the next day? Maybe try talking to him about his plans for the party, show that you're interested in what they're doing, it might be enough of an action to show him that he is acting a little petty about this.
I hope that you can find a resolution to this. And that your friendship doesn't suffer because of it. This is a time for friends and family to come together, and I'm sure you two will find a way that works for you both.
(hugs) and best of luck