Sorry, have been away for a few days but I don't agree with your vet at all, not because it is not a good idea, but a very impractical idea, yes you can feed him first and fuss your Leo and do all the things that may accertain him a higher ranking order in your presence, with his size and if it were with another male or even smaller bitch you could probably push him to a higher rank, but he is living with a GSD bitch.

There is no way your dominant bitch is going to allow any male to out rank her, even if he grows to be as big as a house she is certainly a match for him, Leo's being more on the placid side to the aggressive means she can dominate him easily. It is a no go as far as I am concerned.
The main issue here is his age, he's a big dog, and he is a threat to her and to many, many other dogs, large and giant breed dogs during their adolescent months are more likely to be attacked than any other type of dog, GSD's are also in that same category. He is likely to have many dogs on his walks try to put him in his place, I'm afraid it is the downside to having male youngsters and in particular larger dogs.
It passes.............. I promise it does pass, you need to protect him at this vital time, inside and outside, there is nothing wrong with him, he is doing nothing wrong, it is purely his age, he virutally has a sign on his head right now, saying come and attack and abuse me and put me in my place! Nothing he can do about it, or you, except be very vigilant and protect him, lead him when other dogs are around, be more prepared for him being told off than greeted warmly.
As I said before when he reaches two all will change, he will be an almost adult with a different body language and looked at by your bitch and other dogs in a completley different way.
As for your bitch, she would be getting a real chastising from me whenever she so much as bared her teeth and would be sent to segregation, unfortuntaley she can not be left to sort things out in a doggie way which is the way I would always leave animals to sort out rank, as she bites, so I would take away her powers by not allowing her any growling, or one upmanship, (I would have to assertain myself as the dominant bitch) this is the only thing you can remove from her, it will not de-rank her as your vet suggested, but will control her in your prescence, with the view that you find it unacceptable as her master to show this behaviour.
However, if you do not have a strong character (not everyone does

) where one look or word from you will make her stop and listen, then you won't have the power to chastise a dominant bitch, so just remove her from him, or if it is looking dangerous walk away with your Leo.

I hope that things will settle and that you will cope ok with the situation, it is horrible when things like this happen. What a shame she is reacting so nastily to this young dog. The muzzle is an idea you may have to use.
Wishing you all the best, keep us informed.