Can a father of a 5 month old infant cut the support payment in half because the child has medical bills? He is working and she is still in school?

My granddaughter is 17 yrs. old now. She is a senior in high school. The baby's father is 20 now and holds a job where he works afternoon until evening, approximately 5 hours a day He has the baby during the day (my husband drops her off) until he has to go to work and he and his father bring the baby home. He also has her (overnights included) from Sunday afternoon until Wed. afternoon. The baby has numerous medical bills because of an infection which required hospitalization and well-baby visits and shots. The baby's father was paying $50 (not court ordered) a week for child support, although there are times when my husband & I have to help out financially as well as we want her to finish her last year of school. Now he has cut the baby's support money to $25 because he thinks my granddaughter should help pay the medical bills and this is his way of forcing her to do it. The child still needs formula and diapers every week, and is starting baby food. Can he do this legally?