He needs to do the exact opposite: instead of clearing his mind, he needs to preserve what is in his mind. I often have this problem, and in the middle of talking to my wife about the new drapes I have to sprint off and grab some graph paper, scribble out my thoughts and then go back - otherwise I know I won't be able to hold the conversation. Sometimes if I'm in the middle of a though I don't even realize someone's talking to me for several seconds, and I have to sort of rewind in my head. It's not that I need to say no, it's that I wasn't even expecting something and my brain wasn't ready to switch.