Modifying My Child Support Order

My Son's mother was awarded custody of our 9 year old son. I was ordered to pay child support. For the last two years, my son has lived with me (he is also enrolled in school in my district). I mailed a letter to the court requesting a hearing to modify the child support order and was told a needed to get an attorney. I would like to petition for joint support that that we can split expenses for his care. What are you suggestions?