This isn't good. He doesn't know that you are willing to play along; he thinks it is not consensual and it is a sexual situation for him. It was probably a fascination, and then a compulsion for him. It probably took a lot at first to overcome that barrier of risk - getting caught - even just to take the clothed pictures of butts. Now he is crossing more lines because that wasn't enough anymore. It isn't just about what he is doing, which is creepy. It is what he might do next when this is not enough.