This isn't a surprise, as he is a rescue and the shelter did warn that he isn't friendly with strangers. In part, this is down to a lack of socialization when he was a pup, but you also suspect previous owners were heavy-handed when dealing with this problem and made it worse.
Fortunately, you are aware of the seriousness of this bad behavior and are working with a certified dog behaviorist to put a desensitization and counter-conditioning strategy in place. Although time-consuming, you are making progress and are cautiously optimistic his unfortunate reaction can be improved upon.
Teaching a Doberman to be friendly means unlearning his automatic reaction to the encounter, and replacing that programming with positive and pleasant experiences. This doesn't happen quickly, so expect to dedicate considerable time and patience to this retraining. The result, however, will be a dog that is friendly and well-adjusted.
Key to teaching a Doberman to be friendly is to build his confidence with reward-based training methods and by facilitating positive encounters in any number of different situations.
The basic training equipment needed includes: