Try making it an exciting part of the vacation. "When we're visiting Fun Destination, we get to sleep in a hotel!" Don't focus exclusively on it, of course (who takes vacations just for the mattresses?), but get him used to the idea that it's part of the whole fun vacation experience. I don't know how typical my kids are in this respect, but they find all the "new" furniture in a hotel room to be an incredibly exciting part of the trip.
Bring along a favorite stuffed toy, pillow, and/or blanket that is usually part of his bed, and it will feel much more familiar right away. When you get to the hotel room, point out his temporary bed, let him jump all over it and explore the pillows and blankets. When you're putting him to sleep, do all the usual bedtime routine activities.
By discussing it in advance, helping him "bond" with his temporary bed, and making this seem like a total normal night (just with a different bed), you can hopefully avoid any anxiety when you arrive. And it's also possible to rely on the fact that toddlers are often capable of exhausting themselves thoroughly and falling asleep pretty much anywhere. Our struggle on vacations is keeping the kids awake long enough to get into the hotel bed!