He is likely temperamentally fearful, nothing you have done and difficult to overcome. He needs careful counter conditioning to overcome his issues which may take medication and definitely need an expert behaviorist. This isn't particularly unusual so many have written on the topic. Patricia McConnell's 'The Cautious Canine' would be some help. Dogwise's list on the topic if you'd like to see what sort of thing can help. If you haven't worked much on training dogs a 'project dog' might be a bit much though.