German Shepherd Aggression

Hello! I am new to the forum and am in desperate need of advice. First, a little background: I have two dogs, a 5 year old Husky and a 2 year old German Shepherd. About a year ago, I moved in with my boyfriend. He has a 5 year old Bull Mastiff. All three of the dogs live together in the same house. All dogs are male. My two are neutered; my boyfriend's is not.

Over time, problems started brewing between the GSD and mastiff. The mastiff would occassionally hump the GSD, which would spark aggression in the GSD. He would growl or snap at the mastiff to get him to stop. The humping has stopped, but the aggression on the part of the GSD has worsened. He has attacked the mastiff many times, leaving puncture wounds on his face and neck. It is an awful, high stress living situation that is not fair to the mastiff. I am fully aware of this.

I have taken the GSD for a one week "boot camp" at the facility near our house that trains police K9s. They trained him with a prong collar. While his obedience improved, the aggression towards the mastiff remains. Last night the GSD attacked the mastiff just because I told the mastiff, "NO" for something he was doing. He left a large hole in his ear. This was the worst attack yet.

I am now exploring the option of using an e-collar to further train and curb the GSD's aggression. I would love some advice on how to use the e-collar to ensure that I do not inadvertently worsen the aggression with the use of the collar.

Please, I do not want this to be a discussion of why not to use e-collars. I also do not want people telling me to rehome my GSD. I want to correct this issue and do everything I can do myself before looking into any other options. I would really appreciate any feedback anyone could give me on using e-collars with aggressive dogs. Thank you so much for your help!