One of my rabbit's have lost his appetite ? Any help ?

If you haven't changed food recently and your rabbit has always seemed to enjoy the food you provide up until recently, then its worth it to get him checked out. When my guinea pigs in the past have lost interest in food it has meant they are quite sick. Check around the mouth for any sort of discharge. Otherwise, if your rabbitt is perky and energetic try giving it more of a variety and see if it regains interest in food.