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.
He may believe that there is something better waiting for him.
Unfortunately, this is the kind of mentality that some people who do online dating have.
They have spent so much time browsing through online dating profiles that even when they seemingly find their best match, they continue spending time on the online dating website and effectively spend less time getting to know their best match.