Removing stains from American Bulldog?

I have an 8 month old American Bulldog puppy who was surprisingly easy to crate train. However, he hasn't been so easy to housebreak and even though his crate is the correct size for him, he doesn't seem to have a problem laying in his own waste. He has an outside playtime in the mornings between 5:00 and 7:00, goes to the dog park for 30 minutes during my lunch hour at 11:00, another outside playtime from 4:00 to 5:30 in the afternoons and is out most evenings. He goes to the door when he has to go outside and is never in his kennel longer than 5 hours at a stretch. The problem is that he's white and laying in poop for 2 or 3 hours has left him horribly stained. He get a full bath weekly and wiped down when needed. He is to the point that he's only messing 2 or 3 times a week and gradually getting better, but I've used whitening shampoos with little results. Any recommendations?