[RunShepherdRun]

Hm, based on the available information, I’d also be cautious about calling him 'dog aggressive'. He had been appropriate with your male dog and with other dogs at the shelter, and that matters. Matters even if things are not going well in the new home, and he is not a good fit there. Few GSDs are instantly good with all other dogs in all situations, and step by step introductions can tell us much about what home works for a dog and what not.

Here's a list of some situations where a general category 'dog aggressive' would not tell us all that much:

There are dogs that do very well with dogs of the opposite sex only. How has Charlie been with your female dog?

There are dogs who do well only with other dogs if the other dogs has excellent dog skills, but not with an insecure dog.

There are dogs who do well only with other dogs of a compatible energy level, so a senior with another senior or laid back dog.

There are dogs who get along with dogs of certain sizes, only large ones or only small ones.

There are dogs who are angelical with other dogs until a bowl of food, a rawhide, or a toy is around. Has he perhaps shown resource guarding with your dogs? Is that perhaps why you wouldn't want to place him with a child?

Some dogs do well if they start out as top dog, and a dog who isn't interested in becoming top dog is introduced. But not if they have to start at the bottom of a new pack, or are matched with another top dog, or worse, with an ‘alpha wanna be’.

There are dogs who are ok with other dogs outside their home but don't want to share their home. I know one who has many doggie friends but turns into cujo when sharing the car, but he will share it happily with just two of his dog friends.

And there are dogs who are on edge and grumpy after surgery (neuter surgery in a senior male is not minor) and multiple simultaneous vaccinations (he seems to even have been given the lepto vacc). They long for a room to themselves to recover.

All of the above dogs will show dog aggression in some situations and with some dogs, but can live happily with other dogs.

Pause. And here's what I'd indeed call generally 'dog aggressive':

Some dogs who cannot even walk by another dog without going berserk. That I'd call generally 'dog aggressive'. Often this can be worked with with classical counter conditioning, to get him to ignore other dogs at least for as long as they don't come close. From your description, that does not sound like Charlie’s behavior.

And then there is a very small number of dogs that get eerily quiet and focused when they see another dog, especially a small one. And then go for an all out attack. That is severely dog aggressive.

Also, much depends on careful and slow introductions to the resident dogs at home. Like fences make good neighbors, gates and doors start good doggie relationships. And much also depends on giving privileges to the new dog slowly, to not overwhelm him with novelty.

Maybe you can describe a bit the situations and the kinds of dogs he is having issues with in your home. That might improve his chances. From what you say, he sounds like an adoptable dog, and I hope he’ll find his forever home.