Confused by The Lion king

by Angel
(Saint Louis, MO, USA)

Last December, I adopted a 1 1/2 year old pomeranian from the local Humane Society. I was told he was in too good of a condition to be a stray so he may have escaped. My grandmother is disabled and I take classes online due to medical reasons. My mother, however, works outside the home. When she arrives from work, he gets down into a defensive pose and barks/growls at her. We believe he could have separation aniexty, but is this what it actually is? And in any case, what can be done about it?