He is eating his cecotropes. It is not his poop, but his vitamins. If he does not get it every day he can get very sick and die.
Rabbit House Society explains it better than me: "Cecotropes are not feces. They are nutrient-packed dietary items essential to your companion rabbit's good health. A rabbit may produce cecotropes at various times during the day, and this periodicity may vary from rabbit to rabbit. Some produce cecotropes in the late morning, some in the late afternoon, and some at night."