Rescue dog hyper-bonded to one family member

We got a rescue dog, Caleb, from a shelter over a year ago. He loves men and has bonded intensely to my father (the only other male in the house). The problem is that he becomes very distressed whenever “his” human is out of sight. The rest of us can be home but it makes no difference – he only wants his Daddy. I’d like to help him because it’s not far for him to be so anxious so much for the time (and the whining is more than a little irritating) but we’re at a loss of what to do. Would love to hear any suggestions or what has worked for your anxious pups. Thanks so much!