It seems that they have already told you what they need. You need to obtain a letter from your son's doctor stating that based on your child's disability, you have to care for him around the clock and therefore you would be unable to obtain and maintain a job. However, if your earning potential based on your education, skills and experience is high, then the court may order you to get a job and hire someone to care for your son and pay support for your other son from the left over income.
NOTE: This answer is made available by the lawyer for educational purposes only. By using or participating in this site you understand that there is no attorney client privilege between you and the attorney responding. This site should not be used as a substitute for competent legal advice from a licensed professional attorney that practices in the subject practice discipline and with whom you have an attorney client relationship along with all the privileges that relationship provides. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question.