In response to an acute infection, trauma, or inflammation, white blood cells release a substance called colony-stimulating factor (CSF). CSF stimulates the bone marrow to increase white blood cell production. In a person with normally functioning bone marrow, the numbers of white blood cells can double within hours if needed. An increase in the number of circulating leukocytes is rarely due to an increase in all five types of leukocytes. When this occurs, it is most often due to dehydration and hemoconcentration. In some diseases, such as measles, pertussis and sepsis, the increase in white blood cells is so dramatic that the picture resembles leukemia. Leukemoid reaction, leukocytosis of a temporary nature, must be differentiated from leukemia, where the leukocytosis is both permanent and progressive. This is from http://www.rnceus.com/cbc/cbcwbc.html. Remember there is hot and cold sepsis--- it is possible that you caught him in transition and early kidney failure-- that is because of continued cell break down (highter K) and acidosis from lack of bicarb. Also the severe acidosis would account for needing the higher and higher O2.. Just an idea-- but I have seen patients go from sepsis to MODS (multi organ dysfunction syndrome). Have you found out any more?
PS if he was immunocompromized he would not have been able to increase wbcs so rapidly in my opinion.