The loss of a loved one is a difficult subject to deal with, especially for a child. Many times they do not have the tools to navigate grief.
The most important thing is to talk to them about death in an age appropriate way, with simple, loving and sincere language. Let him know that death is a natural process of life, and is sad, but natural.
If the child is very young, you can use a story, in accordance with the beliefs of the family, to help him understand what is happening. Try to maintain a positive attitude. Help them to remember the happy moments they spent with that special person, listen and let them express their feelings freely. But above all, stay close to them, both physically and emotionally, hug them and let them know that you will be there for them whenever they need you.