It's difficult to see from the photo, but if it is approximately where his tail would be, if he had one, and the problem is black and slightly sticky stuff on his skin and in his hair, then it's probably an overactive grease gland. There is information towards the end of the information in
Boar Care: Bits, Bums & Baths
in the Daily Care and Travel section of the Guinea Pig Guides above to show how you can clean it. I use swarfega now, but have used olive oil (to soften the grease) followed by washing up liquid to wash it off in the past. If it's his grease gland washing it every so often should solve the problem and leave behind clean skin and hair. If it isn't in the right place or the problem isn't consistent with his grease gland then please take him to a vet as it could be one of a number of skin problems.