How long have you had him? Our first rescue girl had separation distress when we got her (howling at the door, whining and pacing), and we did all the usual stuff with her - giving her a bone or kong to work on, not making a fuss, going out and coming straight back in again, creating 'distance' while we were home, both by ignoring and by creating situations where she could see us but couldn't get to us. She got over it after about 3 weeks, but I think a big part of the problem was we were leaving her alone in a strange place. Once she felt comfortable it was her new home (and starting her territorial barking :/) whether we were there was less important.