I guess I'd just ask him what he thinks of it. Remember, some get turned on by something they'd NEVER do. The excitement is in watching something forbidden in their mind. So, maybe just have an open conversation about sexual stuff and mention transgendered sex and see where the conversation goes. His watching it does not mean he's interested in a real way. And agree, do not let him know you have snooped. Try NOT to snoop. Nothing should be secret but respecting space is important in a relationship. good luck
It is also really important that he doesn't think I purposely went searching through his stuff because I honestly didn't I just happened to come across it on the laptop