I just saw this, I am so sorry!
We all have been there, and it is not fun. Last August we took the little family down the street, Mom and two little girls. It was not a disaster, but it was not great. The kids are good kids, well mannered and behaved. Mom not so much. Not bad, just not at all disciplined, and on a Disney Vacacion, you get your carcass out of bed. You let the kids go to the parks if they are "tired" Translation....they want to play on iPads and you want to nap and feel guilty you are not taking care of the kids. You make them sit and eat and stop getting played. Of course they get bored.....who cares? Sit and use your manners.
It will get better, once he figures out that he is not in charge, and that you mean business. Vacations sound like so much fun ahead of time, and they are, but in reality, they require so much more cooperation from the entire family than a 5 YO understands. It is not all characters when you want them, Mickey bars on demand, and walk on rides. I would set an expectation first thing in the morning, explain the consequences in the event he tests the boundary and then follow through. The sooner he is back on "home" rules the better for him and for the rest of you. I bet that you and your DH have been too nice and understanding, and I bet you are not at home.