K.V.
I have never seen a white or really light stool before and have been around babies my entire life. I would take him in, my concern would be that there is a secondary infection somewhere and you would need to have his lungs and everything checked out by a Dr. I wouldn't just call, this seems unusual to me and topped with the fact that he was better and then suddenly worse again. Get him checked in person is better than dealing with something worse later.