He is experimenting with his own socialization right now. He is learning that some of his interactions have positive responses and some have negative responses. Maybe whining does not get him what he wants, but he learns through repetition, so he will try again and again to keep testing your reaction.
He probably is paying close attention to how you socialize and interact with others. Consciously set a good example, sometimes even pointing things out to him. “Mommy said please, so Aunt Jenny passed me a napkin.”
Do you notice that your toddler responds differently to you and your partner? Pay close attention, do you both react differently to him? Usually children pick up on subtle differences in social interactions and respond accordingly.
At 15 months, toddlers are able to start to think in symbolic terms so you might start to see imaginative play. The box becomes a car, or the sofa cushion becomes a boat. Encourage this as it fosters imagination. You may also start to notice bad dreams in association with this symbolic thinking.
Start taking turns during play. This is an important part of social interaction. By physically taking turns while playing, you are teaching him the back and forth of conversation as well as the give and take during social play.