I can't imagine what my reaction would be.
It's really none of my business but since you have posted here I'm guessing you really want the feedback so here goes:
First, let's start off with did he ever want to go in the first place? Was it your idea to go and he reluctantly agreed or did he say from the beginning that he wasn't going? Did he know you booked it?
Next, is he angry about this? I don't live in your house or know your DH but I do know that as wonderful as a
Disney Cruise is....it's not worth endangering your marriage over. I can see my DH saying about a potential vacation (not the cruise but something he didn't want to do) "I'm not interested but why don't you and the kids go?" And I would go, if he was OK with it and everybody was happy.
However if it became a point of contention and agruement and he was upset over it, I wouldn't go. If he is saying "gosh honey, I really want to vacation with you and DS but I just don't want to do the cruise" I would be cancelling in a heartbeat to do something we all could enjoy.
Ultimately it is your decision as to how you handle this. Do you go anyway, do you try to talk him into it, do you invite a girlfriend of yours or a playmate for DS to come along? There are many ways to deal with this. Honestly though, I would try to get to the root of this with DH and make sure you're not rocking the boat (pardon the pun) with hime. It's just not going to be worth it in the end. If he is OK with you guys going and just doesn't want to tag along, then cool, by all means, go without him.
Just my .02$ but I hope maybe it helps with some perspective.