Hey Marisa!
Two things I can think of:
1: Wiggle his casque in that area please. Does it move? There is a bone that runs up the front of the veil and if you can feel it move or it feels squishy, then you might have some slight MBD problems starting.
2: By looking at the previous pics, he may have had a burn or injury to that area and now he's sloughed that part. Now I know he was a freeroamer before you got him and his conditions weren't great so a burn seems unlikely to me, but anything is possible right. The shedding that you had problems with at the beginning is in the same area as that little dip.
Can you get a close up of that area at all?
Cheers!
Trace