It's called transient testicular retraction or ascension, I think. In younger kids it's caused by an over active muscle that causes the testicle to ascend up into the anginual canal. It usually goes away around puberty though it's probably the same thing here. Don't quote me on any of this though.
As long as it doesn't cause pain, or get stuck up there, or swell up or anything it's probably fine just like the doc said. If you're worried or just want more info you should ask to be referred to a urologist, which means one who studies balls. Oh and urine
Just kidding. They just have a lot more specialized knowledge in that kind of thing though.