Why does my second husband change when his son visits over the summer?

I have been married for 10 years. We both have children from a prior marriage. The deal is, during the year, although he is kind enough to my children, he is distant, mostly uncommunicative, and insists on my children working for what they get. My youngest is 17. When his son visits during the summer, he lives in another state with his mother, suddenly my husband is communicative, involved and personable. Mostly in the direction of his son. His son does not try to get involved with my daughter at all and is sometimes distant to me although I treat him like a king when he is here. I feel beaten and manipulated...if he is so unhappy without his kids then shouldnt he just go be where they are? Everything is handed to his son by his mother, and my daughter has to work to earn her car, insurance etc. I dont know how I should feel about all this. I just wish we could all be a family. Any insight would be appreciated.