I can get my in-laws dog to howl. Usually when he hasn't seen us for a while and is excited and happy to see us. I tip my head back and howl and he joins in. Maybe he is telling me to stay away.
It is pretty cute, though.
That’s different. You are part of his “pack,” so he is backing your move (howling) to claim territory for his pack including you.
Or he’s just having fun!