What is the ruling on a young man who participated in the theft of building materials at the command of his employer, and is he required to return the stolen goods?

The conditions for repentance from theft and similar acts include restituting the right to its owner. Those involved in the theft are each responsible for the entirety of the stolen item. Therefore, you must return an equivalent of the stolen materials, or their value, to the workshop owner if your employer does not return them, or you should ask the workshop owner to forgive you. It is not a condition to inform the owner of the right that it has been returned to them; rather, it suffices to deliver it by any means. It is permissible to seek pardon and forgiveness from the owner of the right, even without informing them yourself.