My guess would be EPI. That could also be exacerbating his dog aggression and reactivity.
Are you currently affiliated with a gsd rescue? I would not be placing a dog with those issues on my own. Actually, I would never place any dog on my own but you will need some money to get this dog emotionally and physically healthy.
Ruth & the 4 Legged Rescue Gang
Rafi the malaroo
Gio & Varda, the krazy kittens
....In my heart: