I am not sure I would hang anything around a kid's neck.
If the lanyard the items are on is strong it could strangle him. If it is weak enough not to strangle him the items will just break away and get lost.

I would even hesitate about giving him much pocket trash at that age.
ID and emergency contact card good items for him for sure.

What can you fit in a small wallet?

In the backpack the rain jacket and so on sounds about right.
Maybe some munchies that he likes, a small compass just because and another set of identifiers.
Maybe something like a tag sown right into the inside of the bag with your home phone on it as well.

Edit:
Add some toys to the bag.