H E.,
I am deeply sorry that your dad is not able to share in your life as it is now. It sounds as if you have vivid and genuine memories that would make sharing easy.
My father passed away in my arms long before I met my husband, finished graduate school or had my children. In fact, all of our parents and grandparents outside of my mom are deceased. It is a new form of grief. My son, now at 6, is now able to hear stories me as a baby and child, more and more about my parents. It lends itself to talk about heaven in a concrete form if that exists.
I have digital movies made from pictures, my childhood photo albums still intact as they are an era and provide natural nostalgia. We have saved special items from my dad's world travels and my husband's dad, who passed away when my husband was just 9, left all of his WW2 flags, diaries, war pins and momentos.
My just turned 3 year old daughter was going up to all older gentlemen saying, "Hi Grandma!" This really went to my soul because my father, probably just like yours, would have loved to have lived to be with his grandchildren. I explain that their grandparents are in heaven, and share stories. I explain a little more each year.
My son loves God and Jesus dearly and speaks and questions about death ...It is a little hard for me to hear him because I want to preserve his innocence and he is prone to anxiety but I also want him not to fear death and it is a bridge to our faith.
I would clarify to your young son about others' grandpas so that he is not disappointed and confused. As he grows he will understand more. In the meantime, definitely create that memory box :) and have your mom create a living memory box about her life.
Even an old brief case filled with your dad's stuff plus your favorite diddies from your childhood will be great tools when your son matures past this age of self which is totally a developmental stage. It is still a little bit premature for him to see you other than his life source, support and who he depends on therefore, your wonderful nostalgia and sentiment could be a bit overwhelming. In just a year or two depending on him and how you present it, it could be a lovely shared bond. I would start now with a few pictures now...It is visually easier to grasp that abstract or related stories.
Your mom would be a great person to share those stories too!
Again, as someone who knows, I am sorry your father is not able to share this with you here in an earthly form. I believe our Father in Heaven will provide what we need
Enjoy your son, God Bless