Rabbit cleaning problems...?

My rabbit is now 6 years old and recently we keep finding that he isn't cleaning his bottom end and often his droppings stick to him. Also his back legs and tail area get very dirty. We try combing him and getting rid of any muck on him when we clean his hutch but he gets very dirty quite quickly. Is there a way to make sure he cleans himself properly or any other suggestions to help? Thanks to all.