Will my son pass a 0.5 inch glass marble that he ate a few days ago?

Yes: If your child ingests a smooth walled foreign body like a marble & it made it past the swallowing tube into the stomach, it should easily pass thru the rest of the bowell without problems. Some irregular objects, like staples, tacks, etc may create problems if they injure the area they pass thru but stewed okra fed to the child soon after injestion can encapsulate them and ease their passage.

Almost certainly: A glass marble cannot be digested and will almost certainly pass intact. All the valves in the intestine will push this through. If it does get stuck the symptoms will be obvious. If you are really concerned you can collect the stool starting at about 20 hours to 3 days after the ingestion to see if it passed. If there are symptoms you may need to go to an er but this is highly unlikely.