New dog is aggressive to a single household member

Hi! We adopted a 2 year old boxer-heeler mix a few weeks ago and he's been perfect in nearly every single way.

The issue comes from an uncharacteristic aggression towards a single household member, who I'll just call Ben.

Ben lives on the property but in a separate building, but comes up frequently for dinner, water, to hang out, etc etc.

Almost every single time he walks in the new dog acts incredibly territorial, barking and growling and getting uncomfortably close until he is scolded, when all the sudden he acts completely fine with Ben's presence and doesn't mind him until the next time he walks into the house when the cycle repeats. The behaviour has not gotten any less aggressive since his adoption and I am concerned one of these days he may bite.

Ben has a cat in his building but does not have any dogs.