Hi,
I'm so very sorry for your loss. I lost my Dad a year and a half ago at the young age of 53.
Don't feel sorry for your son, not getting to meet his grandpa, you need to look at this in a new light. Which would be that your son and your Dad already know and have known each other for many lifetimes. Your Dad is always around you and your son and your Mom and other family members.
He died quickly, he didn't suffer, he was a little confused at first but with the light he was able to quickly understand and accept his passing. He's happy and healthy it was just his time to go. He wants you to be able to move forward with your life and enjoy your son. Although you may not realize it your wasting preciuos time and before long you son will be a teenager. This isn't about him, it's about you and the sorrow and guilt you have created from your own grief. Let it go...... And as far as your son filling the big shoes of your dad, he already has.