I would have told my kid that Grandpa was in heaven and the Bible tells us the gap between us and the spirit world cant be spanned (luke 16:26) and dropped it at that
If he persisted I would suggest he pray to Christ, so He could pass a message along
Let the kid read some Hamlet:
I am thy father's spirit,
Doom'd for a certain term to walk the night,
And for the day confined to fast in fires,
Till the foul crimes done in my days of nature
Are burnt and purged away. But that I am forbid
To tell the secrets of my prison-house,
I could a tale unfold whose lightest word
Would harrow up thy soul, freeze thy young blood,
Make thy two eyes, like stars, start from their spheres,
Thy knotted and combined locks to part
And each particular hair to stand on end,
Like quills upon the fretful porpentine:
But this eternal blazon must not be
To ears of flesh and blood. List, list, O, list!
If thou didst ever thy dear father love--
Your suggestion is much better than mine.