You may have to bathe, at least his legs, more often than you would for a blue roan. If he is very young and still squatting to pee, it will stain the legs.
Get a good whitening shampoo, there are many on the market such as Chris Christensens White on White, or Plush Puppy Herbal Whitening Shampoo.
Alternately you can add a dash of laundry bluing to your shampoo to help take the staining out. (Just a bit, depending on how much shampoo you are using and how much of the dog you are washing. Too much and he'll turn purple
Don't worry, it'll wash out)