How does he respond when he is told that when people go to heaven, they can't come back?

Many children at first don't understand that death is permanent. Always in the past their parents have left and they've always come back. It's hard to realize that this pattern has been broken. Sometimes the only thing that can change this is that the child grows old enough to understand death more fully.

If the boy is not in any more distress than you would expect from someone mourning his father, it's perhaps best to just answer the questions honestly and wait for time to do some of the healing. Many children (and adults) also find it meaningful to set up a memorial, look through photo albums, or write a letter to the deceased person.