This is not going to be a topic that will be answered here on enotes. I am not sure anyone can speak with any authority on such a topic. Since you are wondering about it, it might not be very comfortable, but I think you have to openly talk with him about it. There is a good chance that there is no problem, but rather it is something on which he has opinions. There might be some items that he needs to discuss and get out in the open, as well. I think that there is a social perception that if a man is not sexually active, there is something "wrong." While there might be an issue present, there is an equally likely chance that there is nothing wrong. At the same time, I think that it might be important for both of you to talk about this issue because it is impacting both of you. Obviously, you are concerned with it and, by definition, your concerns are going to be his concerns. This is where some level of open communication, or some type of communication facilitated by a third party present, is needed in order to fully understand what is there, why it is there, and how both parties can appropriate it. This is probably the only reason and approach that can be given honestly because it is about him and the only way to understand this is to understand him.