We just talked about this on another post, so I'll re-direct you.
How often should you sheath clean?
To be short and sweet, just do it, and clean a little at a time, do it often and he'll get used to it. Although it's not likely for a gelding/stallion to be seriously injured in or around his...stuff... you don't want to see your Vet hurt. MY Vet was kicked--NOT MY HORSE!!--and the gelding was sedated when he kicked her with BOTH hind feet as she was cleaning his sheath. You would want you horse attended to, especially if he had an injury or laceration there. She was considering giving up treating horses, as a result--same horse owner's mare had previously bit her in the face. (I told her to do so, but she could keep treataing my GOOD horses.)
I routinely clean my gelding's sheaths--always have, I've been kicked at once or twice over the years (warnings) but never hit--mostly they ignore me, but I never linger. The dirtiest sheaths are on geldings who pee without dropping down. The stallion owners here will tell you that their stallions have pretty clean sheaths.