Agree w/ aart.
The best thing you can do is cover the top of your run. Netting or wire will do.
And may I say that Bouncer is a very handsome fella! Best wishes!
Golden Comets are fast to mature and start laying at a younger age than most chickens.
Mine started laying at 18 weeks, which is 4 months. They were Spring babies, though. Time of year makes a difference.
Good luck to you!
Since a large marble is not digestible, it may not leave the crop and it certainly would not get past his gizzard.
If it cannot pass, my concern is starvation. Crop surgery is warranted here.