How do you know that it is his? It could be one of his friends. I hate to say it but at 15 most kids are having sex. I'd just let it go. He might not react well if you approached him about it, not to mention that would be awkward!
I don't know if its normal, but I do know that he easily can go on the computer and see things that are many, many times worse. I do know that I have pulled 8th graders drawings out of the trash can - probably when I was boring - that were similar. So ya, I would let it go.
But, you might also ask yourself - "do I want to be looking in his notebooks?" Tough to answer. Sometimes, just leaving one (that you didn't like) out in a different place will clue him in. Of course, then he will make sure never to leave one out again, or will leave one out to drive you nuts. good luck.
yup I agree with the others , may be best not to look around the bedroom and at 15 he is probably seeing magazines etc dont let it concern you, what does his Dad say about it ?