Here's a thought: Years ago a friend of mine was puzzled by her herd's inacceptance of the new young horse. Turns out the newby was extraordinarily dominant - he didn't need to bite or kick to assert this. He just was.

Perhaps your old guy knows this whipper-snapper is going to be the boss. Perhaps he just thinks the youngster is the wrong gender. I had a male that would not accept male puppies but begrudgingly accommodated an older female.

Yet another thought -- my dogs accept another same sex puppy once the pup is about 12 weeks - as in that is when they concede to play with the pup. This is all bitches.