Not sure what to think really.
I do think that it is something along the lines of like a bend 'or spot, birdcatcher spot, etc. Not specifically one of those, but something that acts similar in that it can and probably will come and go and not be permanent.
I do know that if his sire is really the palomino and his dam was a red dun like you beleive, there is no way that he is brown/bay.
Chiilaa - care to pm with what forum?