I think most of that is normal adolescent curiosity. Probably the best thing to do is talking to difficult child about how puberty and change are normal. I would also address the subject of respect for women and how those women are someone's mother,sister,daughter. Humanizing them may make him see them less as objects. I would also address that pain and sex and naked bodies is not love or a loving relationship.
At this age, his hormones are on overdrive and it's normal and healthy to be curious and want information. As parents, our job is to help direct them from making the leap from curiosity to deviant or harmful.
If your difficult child is reader, I would get him an age appropriate book for boys and puberty. I know both son's used them to get answers when they were too uncomfortable to ask us about it. I really worked hard to keep the conversation ongoing throughout their teen years because one conversation isn't enough.
It's always a worry until you know that your child has a normal, healthy sex drive as a young adult.