A couple of years after he Sego Palm incident, he ate a peach pit.
We were staying in an RV park outside of Chicago, and he started throwing up. When he didn't get any better, we found a vet that was only open all night (divine intervention?). We had no idea what was wrong with him.
The vet X-rayed him, and couldn't find anything. She gave him a shot to stop the vomiting.
The next morning he started vomiting again, and out came the peach pit! The X-ray didn't pick it up because it was the same density as PD. I don't imagine the eraser would be picked up in an X-Ray, either. So any surgery would have to be exploratory, and really scary.
The peach pit was much larger than your erasor, and was eventually expelled. The erasor should not be toxic, and as I mentioned above, should pass through without complications. He's not showing signs of discomfort, which is good, and he's not vomiting, which is also good.
In the mean time, it sounds like you have the fun job of digging through his droppings.