It may well be that he's going back to the bondage sites but not to seek thrills with someone else. Whatever the case, since you two have already been through this in a similar way before, it would seem like a continuation of the same subject and not some new suspicion - so he might well be open to a frank, open talk without a need for spying.
Porn and certain fetsihes have a very powerful effect, and it's not very surprising that he's checking out such material again. But depending on what kind of guy he is, taking the next step and actually practicing bondage with another woman may be more than he's really intent on doing, especially since he got busted by you last time. But since I can't speak to your hubby's personality or character, I'm just guessing.