www.mayoclinic.org
TL;DR Unless he had some kind of intestinal disease or surgery, it's alcoholism causing his Secondary Lactose Intolerance by causing his body to underproduce lactase. In many dairy derived foods the amount of pure m
ilk sugar (lactose) is lower than the amount in m
ilk causing people who have low production levels of the lactose digesting enzyme lactase (in Phil's case due to alcoholism) to be able to eat cheese without symptoms, but not able to drink m
ilk.
Basically if your body isn't able to handle the amount of lactose you put in it it won't get digested in the small intestine like normal and goes to your colon where it causes all the symptoms of Lactose Intolerance. In people with Primary Lactose Intolerance they don't produce any of the enzyme lactase and can't digest any lactose naturally, in people with Secondary Lactose Intolerance they can handle some and it's thought that if you can treat the underlying disorder (in Phil's case alcoholism) this will go away too.