There are several possible explanations
- Maybe the things you want to talk about do not interest him at all.
- Whatever he says will most likely be used against him later
- He does not trust you enough to have "deep" conversations with you
- Finally it is possible you are the unintelligent one in the relationship (with all due respect). As the saying goes "small minds discuss people, average minds discuss events..." Pay close attention to what he likes talking about. Not all guys enjoy talking about politics, sports, gossip or what is trending on social media.