Sheath cleaning....................?

My horse is very shy about sheath cleaning. He'll let me reach in and pull some of the dirt nastiness out, but he won't drop his penis so that I can check/deal with any beans or such things. I've got gloves, very mild soap, horse treats, etc to use for the process, but also an unwilling horse! Awkward question, but how does one get a horse to drop his penis? Again, sorry for the awkwardness, but someone has to help him clean that area. Thanks.