"I'm not sure why she does this, and I haven't even seen her doing it, his forelock just faded away!"
It could be that it wasn't you mare after all. Could he have rubbed it off himself due to a skin problem, bugs, or just being itchy? Does the skin on his forehead look irritated or does he like you to rub/scratch that area? Just ideas.
You could try putting something nasty-tasting on that spot, (like hot sauce), but I'm not sure I would try it, as it might be bad for his hair & skin.
There's a product out there that is supposed to help manes and tails grow back,and it's called MTG, or Mane and Tail Groom. Might be worth a try! Hope this helps.