Dog has attention deficit disorder. Should I ignore or get his treatment?

Its very obvious my 2 year old shepherd mix has attention deficit. When I adopted him, the shelter itself told me, "this dog is not that attentive and has tendency to get easily distracted. Other than that he is perfect. So new training maybe quite a challenge" I wanted to contact the previous owner regarding the dog's poor attention but was not able to trace previous owner. I asked shelter to give me number of previous owner and they said they cannot. Due to his very poor attention span and distraction, it is almost impossible to send him any kind of signal or message. For example when I tell him, "Good boy. Here is your treat", he does not look at me but will turn his neck elsewhere. While walking him he keeps changing direction and starts running. This becomes a pain because i have to keep pulling the leash. When i tell him NO it has no effect on him. He walks zig-zag and does not walk straight. All these things cannot be corrected because of his attention deficit.