Need something to replace the habit, he'll smoke because he's used to it and probably uses it to relax, savour a moment, celebrate, rage etc. Coughing more is normal when you cut down and stop smoking so expect that to get worse. He'll look like shit too as he detoxes, all normal.  Positive reinforcement didn't work for me when I quit. It's a change in lifestyle and how you react to your environment. Like all addiction he has to want to change. Help him by supporting the other things he can do to stop smoking like getting a fidget cube. Giving him all the treats isn't going to help long-term. Once the treats are gone, he'll smoke again.