What is a "hospice foster?" A future therapy dog? Or just a foster dog in recovery?

If possible, you should really look into getting a new trainer. (I'm not sure if this trainer has been assigned to you by the organization you're affiliated with.) This trainer is giving you bad advice. Punishing aggression is not a good idea.

The problem with punishing aggression is that you're treating the symptom rather than the problem. Aggression can have different causes -- for example, a physiological or neurological problem (you should always have a full veterinary exam and blood work done.) High liver values, an undetected but painful condition, or something else could be contributing to the aggression. It could also be genetic.

In most cases, however, aggressive displays are a dog's way of saying, "I'm really uncomfortable right now, and I need more space." It's their way of creating social distance. Rather than punishing the dog for how he feels, we need to teach him that other dogs don't pose a threat (this is called "counterconditioning.") Right now, you're teaching him the opposite -- that every time he sees another dog and gets upset, The Universe causes something bad and scary to happen. Is that likely to make him feel more comfortable about other dogs?

Even if your punishments do succeed, you're essentially "removing the ticker from the time bomb." Now you have a dog who feels just as uncomfortable around other dogs as he did before, but you've taught him that he can't express his feelings, so he's suffering in silence. This can easily create situations where the dog seems fine, and then "all of a sudden he just snapped; it came out of nowhere!"

Another problem with punishing the aggression is that it teaches the dog what NOT to do, but doesn't teach him a replacement behavior that he can do instead. (We want him to be actively prosocial to other dogs, but with many aggressive dogs, the new replacement behavior is doing some easy obedience with the trainer -- we can work up to being prosocial later.)

Aggression is a serious problem, and I highly recommend that you find an experienced positive reinforcement trainer or a trainer who is familiar with BAT (Behavioral Adjustment Training) to help guide you in this process.