It's definitely tough when a parent dies. Kids are usually more adaptable than parents believe.
I'm not sure what your question is- I read that your grandson has been raised by two sets of grandparents and a part time dad, who has custody. He is a very lucky child to have so many people participating in his care.
If you are asking who the child should live with, the father has custody. If he is unable to keep the child full time, then he could discuss other possibilities for the child to live with one or both sets of grandparents.
There is no need for one main home- with one "mommy" and one "daddy"- our world isn't that simple anymore. Kids easily transition between mom's house and dad's house or grandparent's house and parent's house. There isn't a clear answer, nor is there a "right" answer other than to care for the child and make sure responsible adults are guiding him and loving him.
I wish you the very best. If you are dis-satisfied with the current arrangement, set aside some time to talk to the dad and discuss your ideas.
Hope that helps.
Shirley Cress Dudley, MA LPC NCC FACMPE
Director of The Blended and Step Family Resource Center
Author of the book, Blended Family Advice