that I could agree with.. if they are full blood siblings of the same age and one is not maturing there is a chance he is a lesser stag.
Lesser stags also hold themselves more like a hen in avoidance of conflict. If has the cocky stance but slow maturing he may just be a big boy.
When you guys are crossing stuff all over the place you will have variation on when birds come into courage, even when full sibs. You can evem get variation by manipulating environment.