child support calculation

Sorry, I cannot type Chinese in this computer and sorry that I put such a messy case here, please don't curse me for my messy marriages, it really hurts and i just would like to get some legal knowledge here.

I am in California.  I have a son with full custody from prior marriage, son's father is in China and gave me $400 a month now as child support.  Except that, I pay everything for my son and never request any extra dollar from his father, as i know I don't need to waste time and $400 is already a voluntary payment, and my son is a very happy boy, no one can tell his family background.

I have a daughter with current husband, and the custody is 50/50, as my salary is higher than her father, he is asking for child support from me now.  When he and his lawyer calculates the child support, they put my son under my exemption (it is true) however they didn't recognize the cost I pay for my son, originally i put $1500 a month, they didn't allow at all. My question is

1. Whether I can put my son's living cost into the calculation?
2. If 1 is yes, how can I calculate the amount?

I don't have lawyer now, this is the last and only question for my divorce so far.

Thank you so much.