Child in Guardianship Wants Last Name Change

My question involves name change laws in the State of: Kansas
Young child with his half brothers in guardianship care of half brothers grandparent; grandparent not related to younger child. Real father of younger child not in picture; mother is only very seldom in picture, but gave grandparent of older brothers guardianship of ALL the boys. Father of older two has been supporting, taking ALL the boys to places to do things, and even considered as 'dad' by the younger child; the grandparent is even called 'grandma' by younger child. Younger child does not even wish to go with mother for visits when she does come down. Younger child wants his name to be the same as his brothers, grandma, and the father of the older boys; he has been in care of the grandparent since a baby and is now 7 years old. Is there a way to do this?

Was the guardianship done through the court?