Have a helper put a lead rope around one back leg. No pressure on the lead unless the horse raises his leg to kick. It's a pretty easy fix.
I think cheesecloth is best to start a horse with sheath cleaning; it is very soft so no scratching or roughness. Skin-warm water and I put a drop of Dawn in the water. Get up into that cavity in front of the sheath, as it is usually full of gunk. I have only had one horse that didn't like the procedure eventually, and I think he would have if I'd had him longer.
Another thought it having your helper with the lead rope, and all you do at first is just handle his sheath, no cleaning at first. Just to let him know you can handle him if you want to. And he should get used to it and be fine.
My little mare didn't like her udder handled at first. But now she thinks it is spa day.
If I have one that kicks I put a cloth or sponge on a stick (whatever you can find that's light and long enough to keep you out of range) and get them used to that touching them gently. It doesn't hurt if they kick at it so you can keep going until they learn kicking doesn't make it go away and it really doesn't hurt anyway. Make sure you stop when they stand then go back again the next day, when they're better about it do it longer. They all learn it's OK, some even like it.
Yes, they generally respond positively to sheath cleaning. Mine drops his head and his lips quiver, just like when he's having a belly scratch.
It's important to help geldings, and non breeding stallions, keep clean. Rump rubbing can sometimes be a sign of a dirty sheath.
If you have one that won't stand long enough for warm water and soap, but really needs something done, try aloe vera gel. It's sold with the sun tan lotion stuff and very soothing (used for sun burn), and you don't have to rinse it off.