I've had to use the separate-and-reintroduce strategy with Hagrid and Bob. They were best friends as kittens, but after their trip to the vet for neutering, Hagrid went after poor Bob in a serious way! It didn't help that Bob (a former feral) was completely traumatized from the vet visit.

I kept them apart for several weeks until Bob calmed down and got used to me again. He and Hagrid are back to being best buddies--they often walk side-by-side, kind of leaning against each other.

The Feliway should help. I've never heard of using hormone injections--I'd be a little worried about possible side effects.

You might try telling your husband that cats tend to react to household stress, and that things would likely go better if he would be more patient.