With a new dog, even a pretty easy going one, it can take a couple days for them to settle in. That's really common on a first stay. Everything is weird for them, and they just need time to adjust. Continue offering treats as appropriate and try to do the things he likes to do, whether that's going for long walks, playing games, or hanging out on the couch with a pile of blankets. Exercise his body and mind, but don't push him to do things he's not keen on doing, and he'll come around.
It's counter-intuitive, but I've often found that when I take guest dogs to other unfamiliar places, like a friend's house, they bond with me faster because now I am the most familiar thing there, so I become "safe" in this new context. This feeling of safety can then transfer back to my home, and they're suddenly much more comfortable. I wouldn't recommend this with a dog who was more than just a little nervous, though. It's great for generally well adjusted dogs who are just a little unsure of a new environment, but could be seriously hard for a dog who is more fearful.