If he's thinking about an open relationship, how does he feel about you being with other guys while he is off chasing other women?

Maybe he thinks that he is missing something by being in a monogamous relationship?

The best thing is to sit down, talk about it, and figure out why he's reached this decision and what BOTH of you are going to do to with it.

Good luck, and don't hesitate if you need any more advice or support.