I don't think it was fair to bring anther puppy when the other is already scared and upset by other dogs. Pixel needs to be seen by a vet to rule out any medical probelms . Then , the other puppy and Pixel need to be separated or one of them will need a new home.
I am sorry it is not working out for the two of them to be together. However, Pixel was there first, and he should not be bitten by the other puppy all the time. The Doberman is going to get a lot bigger and stronger then Pixel ..