My Mom had stroke-related dementia.
You can use one of these (we used it with my Mom to alert us if she got too far down the hall at night)
Wireless Driveway Alert System
You can place the alert unit on the floor, so that if he gets out of bed and breaks the beam, it will sound. The version we bought (several years ago) was QUITE loud, but my husband "muffled" the sound by wrapping the alarm unit in several layers of newspaper and a beach towel, and then putting it into a ziplock bag to hold his wrapping together. Looked weird and lumpy and silly, but it worked!
You can also hang a Christmas bell on the interior Resort room door that would make noise if he tried to leave during the night. Take along some blue painter's tape (won't damage any surfaces in the hotel room) and hang the bell high enough that he can't reach it, but so that the movement of the door will cause it to jingle.
If he is prone to wandering, I'm sure you have already thought about a lanyard with his name + your name and cell phone number, but, those can be removed. Pinning a name tag to his front works until he removes it. Same thing with "safety bracelets" (used for kiddos and Sr's alike); a determined person will just take it off. Some folks will use a Sharpie marker, and write contact info directly on the skin - and this can work for kids, but he might not welcome that. In our case, I simply wrote the information on a 3 x 5 index card, folded it in half, and safety-pinned it to the *back* of my Mom's shirt or dress, typically at about the level of where her bra crossed her back. She couldn't reach it to remove it, and it was visible enough that if she needed help, someone would see it. Write "If I am lost" on the outside.
If you are flying, remember that airports can be noisy, busy, and scary under the best of circumstances. Some kiddos do OK with that, but others can be stressed out by the loud noises (think jet engines backing away from the terminal, the beeping of the carts, etc.) and the stress of TSA checkpoints, so be prepared.
