S.H.
You can tell him that She will be playing with the dog in heaven. Try to explain that when people get older, sometimes their body gives out but their spirit lives on in heaven. It is hard for anyone one to deal with at any age, explain that it is ok to be sad that she is no longer with us in body, but ok to be happy that her spirit lives on and his doggie now has grandma to play with. I am sooo sorry.