That's insane. It's one thing to let a little BO slide every now and then, but this kind of stink is way more offensive.. Especially for someone having sex often and getting all those fluids trapped under there.
I can't imagine going even a day without cleaning it, let alone a whole week. Even if you ignore the horrendous stench factor, I don't get how he's not constantly itchy and uncomfortable from it being so dirty/bacteria-infested all the time.
If he's been putting you through this for a while and won't even consider changing his ways after you've tried honestly talking with him about it and telling him how it affects you, then I can't see this relationship working out too great unfortunately.