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This is a fairly common issue with Min Pins, as their tendency for dog aggression is rather common...and they typically do no care how big or intimidating the other dog may be. Min Pins actually have no genetic relation to Dobermans, but were actually bred selectively from terriers and chihuahuas to give them the look look of a tiny little Doberman...but with the feisty temperament of the chihuahuas and little terriers they were originally bred from.
You did not mention if he is neutered, and this is a very important piece of information. Intact males of any breed are prone to exhibiting territorial and dominance aggression toward other dogs. Therefore, if he is not neutered, that would be a very good first step. Eliminating the testosterone may go a long way toward evening out some of that vinegar.
From a training perspective, I would suggest a gentle leader, as these are very effective in reigning in dogs that are prone to bolting after other dogs, cats, or any other critters they may get passionate about. They can be very effective in helping you control him when other dogs are near. Here is an example of one:
Regarding direct training, this can be challenging to simply post over an online consultation. In these circumstances, you need to be able to establish yourself as his leader, and it must be made clear that he is not to deviate from this, nor his eye contact with you, no matter who may be around...but this is far easier said than done. Therefore, I would suggest insisting the assistance of professional dog therapists. In this regard, I have enjoyed allot of success with Barkbusters, who have local therapist all over the English speaking world, including the US and Canada. They come to your home and guarantee their results. Here is their website: