It depends on how new of a relationship it is. Out of the gate, I don’t think he owes you any details. However, if he has expressed interest in exclusivity or any level of emotional and/or physical commitment, and longevity, he needs to spill the beans. Communication is key in any relationship. He shouldn’t want to keep you in the dark. Make it clear that you’re not asking for details with the purpose of judging or scrutinizing the situation. Use your ears more than your mouth the first time you talk with him about it. But above all, listen to your intuition.