I'd say that if you agreed to be open and honest and feel that he's doing something hidden and dishonest, then you have reason to talk to him. But, since he can't prove that he's not doing what you fear, it might be more that you two figure out what it is that you're afraid of. Pull him in as a partner in the conversation, rather than working from a place of fear, perhaps.
Since you name his ex(es), do you fear that he's trying to rebuild something there? Do you know enough about their pasts to know why they are no longer together and not presently together? I'm the kind of person who likes to know a person's past, so I'd ask about the person, regardless of what's going on in the present. Might give you information about what's going on in the present, too...