German shepherd does not care about treats or listening to me

I have 2 Shelties and a GSD. My Shelties are very attached to me and off leash trained. One of them is extremely consistent and intelligent and follows me everywhere, but his brother is more stubborn. I trained them with food rewards and praise, both of which they’re very receptive to.

I adopted a GSD who is the polar opposite and confuses me. His breeder had him off leash trained and he was in shows. When I got him I tried to get him to sit and he just stared at me? I’ve tried all kinds of food rewards and he does not listen consistently, especially if something better rolls along. I sometimes feel like he doesn’t like me, but he cries when I leave the room.

I brought him to a trainer who recommended using a prong collar. She was even surprised by how poor his attention is. I need to get him to consistently follow 3 commands before we can try an e-collar but I’m having a really hard time even with being consistent.

For instance, when I tell him to sit, he’ll stand up before being told to break. No matter how many times I lightly pull and say “wrong”, he still stands up early. It’s been like 2 months, so I feel like I’m not communicating correctly with him, but I have no idea how to fix it. Even if I try his absolute favorite treat (Beggin Strips) he’ll still stand up early.

He’s a smart dog and I want to engage him. He’s very good in public, walks pretty well on the leash, and is generally reserved and quiet. I’ve been wondering if he’s depressed and misses his previous owner and is just never going to listen.