He's the hotot, right? (Looks myself) yep! Okay there might be a few reasons he's got a dirty bum : Overweight or sick most commonly.
*What's his diet been lately? Anything new? He might have had a bit of diarrhea due to a change.
* Have you noticed him eating his cecals? Sometimes heavier bunnies cannot reach their anus to directly eat the cecals and then they end up with dirty bottoms because they sit in it.
* Drafts and severe changes in environment can spark a stress reaction which gives them a touch of diarrhea. Less common however.
*How is Speckle? Is she the same? THe thing wit bonded bunnies is that if everything is the same in diet, activity etc with both and she is not dirty that it might be a sign that he needs to be checked out at the vet. Perhaps not though, watch his litter box for poops. Examine the poops to see that they're dark colored, round, and erm.. Well moist- looking as opposed to dried out looking. You don't even have to touch them to see this. Looking at poop is a basic thing to check, try to have him use his own for a day or something. I don't know how hard that might be. Also, check his urine for sludge. It couldn't hurt to know if he's maybe got too much calcium going through him and a possible blockage.
*As for cleaning, do you know what hibitane soap is?? It's a dilute chlorhexadine solution which acts as a harmless cleaner and anti-bacterial. Vets sell it but I don't know if you can buy it elsewhere. You can dilute some iodine to keep the fur clean but make sure you rinse well!! It stains and shouldn't be ingested. Also make sure no iodine gets into his orifices!! And finally, make sure he's dry after washing his bum. When bunnies get wet they can get even sicker drying out. Try not to have fans or drafts near him as he dries.
*finally- good luck!! Hopefully it's nothing serious. Sometimes it's hard to backtrack monitor diet and environment. Keep him eating and watch him is all for now