You should confront him about using online dating sites if this is a person that you are in an exclusive relationship with.
If the both of you had previously agreed to be in an exclusive relationship with each other and you have discovered that he is using online dating sites, this is something that must be addressed.
Even though he may tell you that he has no intention of meeting these women and all he is doing is having conversations with these women to kill boredom, that is still not acceptable.
Even if he may never meet these women that he is interacting with online in real life, he is still at risk of developing an emotional attachment to one or more of them.