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?

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.
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.