If he is a baby chick he needs to have this removed or he could die if it blocks his poo from coming out. This is called "pasty butt."
If he is older, it is always nice to get those poo balls off them. I recommend soaking the area with warm water (be careful to support them in the water as they fall asleep sometimes). Then pick it off as it softens.
Be SURE that you put him in a towel and dry him off and back under the heat lamp he goes. Or if he doesn't use a heat lamp (you can do this even with the heat lamp) dry him with a hair dryer but be sure you don't burn him.
I've even stuffed little baby chicks down the shirt to warm them up after baths!!!!
If he's almost grown up and it's cold outside I'd use a hair dryer on him. Chickens can catch a chill easily.
It will take awhile at first I couldn't do it and had my dad 'de-poop' the chicks, but I got it pretty quick. We had an old cloth and would soak it in warm water and pick up the chick. At first we'd hold the cloth there for a bit and then start to pick the poo off. It can be a slow process, but you'll get it off in time.