well he's going to be more irritable, he'll be detoxing so it'll cause headaches and frustration. He also may gain some weight since nicotine tends to suppress hunger. Take it very slow don't just cold turkey it, he needs the nicotine right now so I'd suggest maybe brining the habit down a few cigarettes a day to help himself get used to only allowing it so many times per day, or just during highly stressful situations. As well he may want to try the patch or gum to assist in the cravings and assist in not smoking so much, some people use gum as a new addiction but to be honest it's better it helps clean the teeth as well. Be prepared there may be slight moodiness since addiction is a very hard thing to break. It wont happen over night either so try to be as supportive as possible without being too pushy because that will just cause him to continue to smoke more.