Can grandparents be granted legal custody of a grandson?

The answer is yes. If the parents are involved in an underlying family law case, the court has authority to grant custody to a third party under the custody statute, if there is a finding that both parents are unfit. If there is no underlying family court case, you could try filing a CHIPS petition on children's court or a petition for legal guardianship in probate court. Both parents would have to be noticed of the court proceedings, whichever forum you select and would have the right to object to make you the guardian and/or custodian of their child.