Some dogs are just so independent that they just don't come when called. We have 2 doxies and they have very different personalities. One always immediately comes when called. The other comes on her terms. I know she doesn't have a hearing issue because you can whisper "who wants a treat" or "who wants to go" and she in there in a flash.
Your new family addition is so cute. Thank you for adopting him into a loving home.