Well, if you multiply 16 oz (1 lbs) by (1/3). You get 5.28 oz for 3 slices. If you divide 5.28 by 3, you get 1.76 oz per slices. This means that he can eat 2 full slices of turkey and half a slice in order to eat less than (1/4) of a pound.
When your losing weight after consuming more calories, you need to increase physical activity to increase the number of calories your body uses for energy.