How do you build white cells?

A drop in blood cell counts is but expected with chemotherapy. I'm sure the doctor does all necessary measures and makes his decisions carefully based on risk-benefit analysis to see whether your son is fit to complete his chemotherapy.

Meanwhile, what we can do is to help your son recover from the expected side effects. Provide plenty of rest and avoid getting him fatigued. Make him as comfortable as possible. Guard him away from all sources of infection by limiting the caregivers inside his room to minimum and cleaning the room as necessary. No fresh plants or flowers inside the room, and make sure all his meals are thoroughly cooked, as raw vegetables and fruits may harbor lots of microorganisms. Give him a balanced diet appropriate for his age. Prevent bodily trauma and avoid those situations that make injuries likely. Appropriate doses of Vit C and multivitamins might help as well. Lastly but definitely not the least is the need for emotional support. May your son get well soon. My prayers for him.

Edit: Although a drop in blood cell count is expected, sometimes doctors prescribe drugs like Epogen and Neupogen to help increase the blood cell counts. I agree with the one below me that you may want to discuss this with the oncologist.

That is a very low white count. I am surprised that your oncologist has not recommended Neupogen. Neupogen is one of the commercial proteins called granulocyte-macrophage colony stimulating factors (GM-CSF) used to treat neutropenia (low wbc count). Ask your doc about this protein for your son. Good luck.

if thats your concern you can make your son drink vitamin c but only upto 1000iu maximum in one day most likely this will be 2 tablets of ascorbic acid vitamin C's

or you can consult your doctor for your concern