Simply don't respond when he doesn't answer you. If you ask him if he wants chicken and he just stares into space then don't give him anything. Maybe giving him choices that require a response. Or maybe he just needs more time to think. If he's staring at you maybe you could say something like "Do you need a little time to think about that?" and see what he says. Does he have a cold or allergies? He could have fluid in his ears preventing him from hearing everything you say to him. Try making a point of making eye contact every time you talk to him and see if that helps.