By the age 13 I was already curious about sex like many on this forum- there wasn't any internet for me to look up porn with, but to be quite honest that wasn't my primary objective- I was seeking answers to all of the questions I had about sex- much more than seeking a depiction of my sexual fantasies- I would suggest that you make a book readily available for him to look at privately, which would contain the answers to any questions he may have- The questions I had were vast- and my parents had a book called "human sexuality" which was actually a college textbook from one of my mothers nursing courses (there was a note on the inside cover). I found it to be very useful and educational- My first "pornographic" experience was from this book as well- There were illustrations of various sexual positions, coupled with a description of the techniques involved and the various sensory experiences one would feel- The book should be a good one- with unbiased information about safe sexual methods and practices, reproductive organs, menstrual cycles, sexual deviance, deformations, basically anything you would expect a college couse book to contain- He will find the book if he it's around, and he will read it- and by the time he is "old" enough to take sex-ed he will have a college level education on the subject- And he will be able to make informed decisions pertaining to sex.
just my thoughts on the subject-