My son is 21 now, I've never received any money from his father, but now I found him would I be able to collect back pay child support for the 18 years, he didn't support me or my son? In state of Ca? We received assistance for total of 5 years.

Was a child support order ever entered through the courts? If not, its unlikely you can collect back support. If there was, then you can move to collect the support arrears.

The state can try to get reimbursement from him for the assistance you received. But you wouldn't get any of that.

Did he flee the State to avoid his responsibility and did he make an effort to remain hidden through the years?