Mother Pays Child Support, Needs Help!!

I am the mother of two children ages 13 and 17 1/2. I pay child support to their father. My question is, my son will be turning 18 this September and how do I go about taking him off the support order and lowering my monthly payments? My income is only $400.00 per month and I live off of my current husbands income. My exhusband makes approx. $2400.00 per month and I am paying $350.00 per month in child support. I live in California and cannot afford an attorney, any advise would be greatly appreciated! Thank you, Melinda McClain