You need to soak his butt in a nice warm bath to loosen whatever's there. Then you can clean him thoroughly - and dry him really quickly so he doesn't get cold, and you can see what's really there. Could it just be poop attached to feathers?
I have been prepared every time i check the chicks to clean the butts of each one I handle, need it or not. Cleaned every butt every time with warm water dipping a paper towel and pressing/wiping gently on their butt several times will loosen anything and keeps them clean; you must change towel every chick.. No pasty butt problems to speak of and drying is easy with a paper towel.
Only needed to worry about this for a short time, they grow extremely fast.