I could share some horror stories with you also. Besides the bead in the ear, the black-eyed pea up the nose, the metal ballbearing that my daughter decided was something she wanted to teeth on (and swallowed, I won't tell you that story)there are only a few boring stories, lol. We made so many trips to the ER that they knew my daughter by name when she walked through the door!
Seriously, my daughter did this until she was 6. I always watched her like a hawk! She and I talked often, and after I put things together, it had a lot to do with her ADHD. She was so busy with her mind and her hands, and it always happened when she HAD to sit still. The dinner table, the toilet, the school desk...all places where she wanted to wiggle and wasn't allowed.
When I detoxed my house and removed all the synthetic chemicals that were causing her ADD/ADHD, the behavior stopped. I don't know if that is what is going on with your son or not but detoxing is never a bad idea. Let me now if you want more information. I love to share what I had to struggle to learn :)