Hi there, I am very sorry to hear about the troubles you have been having with your mastiff. I myself recently lost my neapolitan mastiff at 8 years old and he was irreplaceable. I have a great fondness for the breed. And after seeing him defend my apartment by literally jumping through the window when someone tried to climb my fence, I know first hand the awesomeness and scariness that a protective mastiff can be. I will try my best to help you but given that you are already working with Dr. Deporter, I do not know if I will have anything to add.
First, if I may, I would like to get a little background.
Has he always been this over protective or did it recently escalate?
You mentioned picking him up from a playdate- does he do well with other dogs when you are not around?
Can you pinpoint the escalation to a certain event or trauma, however slight?
Is he neutered?
Is he just as aggressive when you are not around? Is he just as protective over other family members?
Does he accept people into your home?
I can look for trainers in your area that have a track record of dealing with aggressive giant breed dogs. I think that it is fine to want to try to rehabilitate him. You obviously care for him so very much and if he goes to another home, he is likely to have the same issues with time. And if he does, his new owners may not be so astute and dedicated as you are. Any dog with aggressive tendencies is a lawsuit waiting to happen but you are protecting yourself by trying to treat him appropriately and re-train him.
And of course, Im sure Dr. Deporter recommended using a basket muzzle when i npublic for everyone's safety. No one can offer any guarantees that anything will work but if you are not ready to give up on him then you have no other option but to try.
Unfortunately, I would have to respectfully ***** ***** Dr. Deporter in that if you decide you are not able to address his issues and rehabilitate him, I think the best thing to do would be to euthanize him rather than rehome him as he could potentially be a danger in his new home as well. The caveat to this would be if the breeder is willing to try to rehabilitate him and assumes the liability for him.