Poor love. I guess he's got a white coat. Rocky has this a bit with the white fur under his chest (when he sits in it) but I find baths get rid of it. Sorry I can't help any more. I'm sure someone else will though!
Might be worth e-mailing Chriisie at Gorgeous Guineas (www.gorgeousguineas.com) as I believe she does make things up as a special request so maybe she will know just the thing to get out the stains safely for your piggy.
My George does this. He's a lazy dirty boy who just pees where he sits and doesn't move. He has a white coat that is stained yellow at the back :
Every 3rd day I wash his bum in a weak solution of baby bath and warm water, dry him and then slather the bits that get wet with vaseline to protect his skin. before I started doing this his skin was so bad it would come off in lumps. Since I've been doing this there has been no problems at all.