dried poop on long hair cat butt

i have a cat n he has dried clump of poop on his butt.problem getting it off is he freaks out and attacks me when i try. how can i get him to either fall asleep or relax enough so i can take care of the problem? pleaseee someone help me out.

Firstly is he overweight?  If he is he might not be able to clean himself properly.

Secondly are his stools soft?  If they are that might be part of the problem.

If neither of the above are an issue then trimming the fur there will probably solve the problem. 

Just wait until you catch him asleep, take the scissors and trim it off.
 

If you use scissors, try to use children's scissors or ball-tip scissors. Be very careful. Feline skin is thinner than canine skin. It is easy to cut the skin without realizing you have done so.

I had this problem with my cat. But he didn't mind water so we sat him in a tub of warm water until it was soft enough to get off, then kept the hair trimmed.
 

Get him and put him in a bath so that you can rinse him thoroughly and if it is not possible the cut it with a small scissor so that he will not be afraid of.