Well, it is very unlikely that your 6yr old does have any kind of retinal break.
Assuming that he doesn't have any other ocular or genetic disease the only thing that could raise his risk factor would be high levels of short-sightedness.
Now this is also unlikely in a 6yr old but you did say that he some family history and is getting glasses, do you happen to know the numbers?
I think you should have been given a written prescription as you do in the UK.
Lots of people do have pigment at the back of the eye (think freckle or mole) but not many types of them would be there at age 6.
The ones that do can have a fairly variable appearance and examining a 5yr old is not always easy.
The odds are very much on your side and you could be further reassured by noting the glasses being prescribed are not for high levels of short-sightedness.
If you have the numbers, I could do that.
But in general, you should be aware (for yourself and your DS) of the symptoms of a retinal detachment.
A sudden shower (or change in) floaters (little black/grey spots or strings in the visision), flashing lights being seen for no cause or a shadow coming over the vision from the side.
These would require urgent ophthalmic care.
Hope this helps you not to worry a little.