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.