[I Luv My Sami]

Sam is 2 years old and always has been a good boy but the last couple of months when my boyfriend gets ready to leave he acts like he is going to attack him. He barks at him really loudly and follows him all around the house and acts like he is going to bit his ankles. ( He doesn't bite him) And then when he does leave he barks even more and jumps at the door. Apparently he only does this when I am home. And he doesn't do it every time either. I have tried holding him so he can see him getting ready and tell him quiet and make him sit nicely and be quiet before I let him go. I don't know what to do. I have been able to train Sam on any other thing but I am having a really hard time with this weird and naughty behavior. Any advice?