Boy Dog Behavior

This is my sweet boy Grady. He is a little over 2 and a half years old. You couldn't ask for a sweeter boy. He knows his place in the house (girls rule). I will preface this by saying, I have never owned a male dog before. I own three girls so I am unfamiliar with the male behavior. He has been acting pretty silly lately (the last two weeks). He chases Dixy around and acts very amorous with her. He licks her ears, bites her neck, and yes air humps her. Dixy is spayed and 4 and a half. It makes me laugh really. He is so persistent. That is until Dixy has had enough and tells him off. The interesting thing is that he only stops briefly. It is like he has a short circuit with this lesson. The funny thing is that he doesn't even think about behaving like this with Sara (his mother who is also spayed). I am really fascinated with the differences in their behaviors.