Do you know what my dog Ceaser is mixed with?

I got my dog Ceaser from an old man so I don't know what kind of GSD mix he is.

He's pretty big - about 28 inches - and last time I took him to the vet he was 120 lbs. But I think he might have gotten bigger(he's very heavy and couldn't fit on the scale).

He's very smart and I think he can understand people (one time I told him that he's lucky and can do what ever he wants and he looked at me then down at his private part then walked away... he's neutered).

He is playful, full of energy, barks and howls all the time at night and when we leave, and protective.

I'm curious to know what kind of dog he is exactly. Any ideas?