When a child is being kept by the grandparent is the grandparent's income used when calculating the father's child support payments?
The parents are responsible for child support, not the grandparents. If the grandmother has legal custody the child support payments should go directly to her via a court order. All states in the US have child support guidelines, based on income and other factors, that must be used to calculate what the non-custodial parent(s) must pay.
It should be noted that if the child lives with her grandparent that grandparent is already contributing to the child's support above and beyond the payments made by the parent(s). Child support payments rarely amount to enough to completely support the child.