Put spyware on his computer or phone. Just test it first on your own to make sure it is not detected. Anything he knows about will be useless. Good spyware will email you screen shots. From that you will be able to see what he is accessing, passwords too.

Tell us what he is using to access the web and you will be able to get more detailed help.

3 adult children
Divorced - he was a serial adulterer
Now remarried, thank you MB
(formerly lied_to_again)