Rabbit keeps getting straw and mud stuck to the underneath of his tail?

Does he have enough water? If so, then he is okay. For my rabbit I had to wipe his bottom every time he went number 2.

^^ Maybe give him more hay, it can help alot. My 2 bunnies sometimes have this problem. Putting them in a couple inches of warm water (No shampoo or soap)for a couple of minutes helps loosen the poo, it should come of easier after this. Usually works a treat for my bunnies (: