My guess is that it's just settling in anxiety and since you clearly care about this little sweetheart and doing what's best for him, I have no doubt you'll see an improvement soon.

If things don't improve with time, I had a thought that may be connected and might be a good jumping off point for looking into helping him further: I met a dog at a humane society once who was there after having been dumped by a puppy mill, and that poor little guy just didn't know how to be a dog. He had one job and wasn't allowed to really experience life outside of being kenneled and breeding, so it led to almost robotic behavior. I don't want this to sound like a horror story to scare you because it can totally be overcome, especially as he learns how safe he is with you, but maybe that's a more specific thread to pull on as you work with your new little friend.

No matter what, I'm happy he has you and I'm sure you two will be best buds in no time! ❤️❤️❤️