This seems to be a common theme here. Don't beat yourself
up over it. Turn it positive by not letting it not happen again.
I'm not sure the age of your son but you could use this as an
example of the dangers and realities of nature. Try to make sure
he doesn't blame the animal and assure him you will do your best
not to let it happen again. Things like this can cause anxiety in
children but you can stop it quickly.
This has happened to a lot of people here. Sometimes it's their dog.
Sometimes it's someone elses. I let my dogs run free with my chickens.
The dogs aren't aggressive but one does like to play with the chickens.
She can easily damage a chicken while playing with it.
Remember, make this a learning experience and try your best not
to blame yourself or anything. I know this is easier to say than do.
Vent it here. We're listening.