Just Adopted a Black Lab/Pitt, he's so afraid of everything

Hi everyone, I recently adopted a 9 month old Black Labrador Pitt Bull. I did not adopt him from a shelter, I literally found him wandering the woods eating sticks and his own feces while on a walk with my other dogs. I looked closely checking for signs of rabies and saw nothing. I immediatly brought him home, bathed him, fed him and immediatly took him to the Vet for a raibies vaccination and check-up. For the first couple of months he was rather mute and frightened of even the slightest noise. He has been a member of our family for over 3 months now, and has grown significantly so I want to begin training him. He doesn't do too well with treats, only when I gently put them on the floor or slowly beckon him over.

However, he's still very frightened of things and doesn't come to me when I call his name ( Manchester). He runs and hides from me if I move a chair or close a door too hard, his ears and tail are down most of the time. However, he is playful and loves to rip and run through the woods on walks and outside on our lawn. He has a very deep, nice bark that he uses when he hears the door open, or people's voices next door, very protective.

I really want to start training him, but I don't want to pay 100s of dollars for a professional trainer when he seems to have some psychological issues that I can't seem to get past him and would completely blow any training I want to give him ( he rushes away from me if I come up on him too fast, which in my mind is rather slow and gentle). What should I do?