Just politely say "I don't want to talk about that, let's talk about tv shows, what's your favorite comedy right now?" (Or whatever other mutually fun topic you want to talk about.) If he tries to keep talking about the topic just repeat that you're not gonna talk about that and offer a second new topic. If he still wants to keep talking about it say "alright I'm gonna go talk to someone else about a different topic, I'll catch you later", then walk away from him