You may have to give him a "butt bath", but I would honestly look to the cause of the problem first. A young rabbit should not have feces matted into his butt.
Common causes of this are:
-- obesity (rabbits unable to reach their hindquarters to groom; can also lead to dietary problems when they are unable to consume cecals)
-- dietary upset (soft stools sticking to the rear unnaturally; abnormal cecal production, etc)
-- dirty cage (litterboxes should be refreshed daily and completely emptied at least once a week)
-- other medical problems (pain or discomfort while stretching, bloating or gas, all can lead to a disinclination to groom the hindquarters)
-- advanced age (often older rabbits can become more slovenly in their grooming habits and may require help from their human friends)
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry