Pooey problem!

I went out to my rabbits this morning and I was just having a hug with my rabbit who is soon going to be 10 Years old and a lump of poo was attached to his bottom and fur. I know older bunnies sometimes don't wash their bottom so I really don't know what to do?!

Please help.....

My elderly buck sometimes does this but I know he has arthritic joints and struggles to stretch round the rear to clean himself. I ensure his fur is kept short in that area and keep a close eye on him.

Ok. But what should I do because I don't want him to get an infection