Please do not use mineral spirits to remove poop from a chickens behind.
What you need is a tub of warm soapy water, gloves, scissors and a pair of pliers.
Soak him in the soapy water for about 10minutes. Work the water into the feathers using your gloved fingers. For hard balls of poop, use your pliers to crush/break them up, this makes it easier to see and get them out. Cut away any remaining poop in the feathers is need be.
Inspect his skin closely for any irritation, wounds and cuts. Pat dry.
If he is impacted with poop - separate him out so you can monitor him. See that he is drinking really well - if he seems constipated add a small amount of molasses to his water one time - molasses has a laxative effect. After he is drinking, then offer food.
When you can, post some photos of him. If he has not been eating/drinking for a couple of days, he may have more going on.
Keep us posted.