This is very common for guys. the best thing to do is not even tell him you noticed because he might feel embaressed, or he could think you were spying on him, etc... IF you feel like you would like to talk to him, don't even tell him you saw him. start the coversation along the lines of growing up, changing, and all that stuff. Then just remind him to make sure he clears his history if hes using a cmputer and all that so that siblings or other people don't find out what hes up to.

Im sure you are a great dad and will make the right choice.

Roman.