Another thing you might consider since the diaper rash is not clearing up is that the yeast is now in the intestines. A simple urine test can give the results of that. If that is the case, you can try Grape Seed Extract drops, or Diflucan, or Nystatin (perscription). You can try yogurt to help clear this also, this can help to re-establish good gut growth. Grape seed will kill the Canditis. Nystatin may take longer than several weeks especially if the candidtis really has a stronghold.
This is a very common problem with children who later develop autism. Is the child vaxed?