Does Suguru even have voice acting?
Well, here's an album of all the cast credits from the first 9 episodes of Non Non Biyori: link. None of them list Suguru. 

At minimum, then, we know that he hasn't had any lines so far. Maybe we'll have a big reveal in the final episode or something that Suguru actually does speak, but for the time being, I think it's safe to say that he doesn't have a voice actor.