Hi,
Well that doesn't look nearly as bad as I was imagining.
That is just the top layer of the scales coming off not the actual scale itrself from what I can see.
That should be fixed after the next shed.
As to what caused it however I would still keep trying to think if the humidity was off during the last shed, he can scrape his back on anything rough or if you had applied anything oil based to his skin at any point.
But Unless the picture doesn't capture the problem I think there is no real need for a vet visit.
dr del