My thoughts:
If you get caught snooping you will lose all trust and credibility.
What can you do with what you find out without giving yourself away?
I would tell him that he has broken trust and so you will need to watch him carefully - not snooping, it's out in the open.
You can discuss everything that goes into making a responsible adult - how to treat everyone (not just girls) with respect, how to treat himself with respect, how to be responsible in school and later in work, etc. etc. etc.