A.S.
It is definitely possible for him to shove it up so far that you can't see it. I'd advise taking him to the doc to make sure it's not still up there.
Here's a little anecdotal story for you... When I was about his age, I shoved a piece of cotton up my nose. My parents didn't notice it for weeks. Eventually it started to rot, and that's when they finally knew something was up - I stank! By the time they got me to the doc, I had a horrible sinus infection/ear infection/tonsils were inflamed, etc. I ended up having to have my tonsils out, tubes in my ears, the whole deal. I still have the scars inside my nose and often have to tell my tale to any new doctors that I see.
So yes, I'd highly recommend you'd get it checked out, just to be safe. =p