I think that you need to trust your instincts and his. Dog's intuition is so keen that they are able to sense when something isn't right. Bear has done the same thing before but I trust him and know he is just protecting me. He usually loves everyone and isn't a growler or barker, but a few times it has happened so I knew something had to be up. IMO, I'd say watch him, do a proper introduction, and just monitor the times he does act in that way. I bet he did sense something so praise him for being such a good protector of his mommy.