My son is 20 months old. he had severe jaundice. he can not stand without support and his cognitive abilities are below his age. what is the problem?

May be the bilirubin: In the newborn period the brain can suffer if the unprocessed bilirubin levels get high enough to cross into the brain & damage the nerves.The term bilirubin encephalopathy is used to describe a form of cerebral palsy caused by this process. An evaluation by a developmental pediatrician or neurologist can help you sort out this problem. There other causes of these sx.

As Dr. F said, there: May be an inborn error of metabolism or metabolic syndrome underlying both his severe jaundice & developmental delays. See your pediatrician, who may consult a pediatric endocrinologist, metabolic geneticist, neurologist &/or developmental/behavioral pediatrician. Also, enroll him in your state's early intervention program for assessment & therapy, which are free & don't require a diagnosis.