Emergency Essence is one of the commercial combinations of the Australian Bush flower essences (there are almost 70 individual essences, 7 of which are in Emergency). It's kind of like Bach's Rescue Remedy but more powerful and more effective. It's available in a few different forms. A dosing strength dropper bottle, a stock concentrate bottle (with which you can make up lots of dosing bottles, with or without some alcohol preservative), a misting bottle (which includes ingredients not appropriate for cats - if you want to spray you can transfer dosing strength into a small spray bottle), and a cream. I recommend the stock concentrate as the most versatile.
I have the kit that includes all the individual essences so I make up my own combinations. 7 drops of stock into a small amber dropper bottle, fill most of the way with a pure water like distilled, and rap the bottle against your other palm a few times to "set" the energy. I usually dose by applying a few drops to the top center of the head (a chakra point). It's an energy medicine. Safe for cats, unlike essential oils.
There are more effective essences than just Emergency for depression but Emergency is a good start and cheaper than purchasing individual essences. I probably should not post a commercial source of the essences, but if you're interested contact me privately.
Grief is very real in our pets, either because they don't understand why a friend is gone, or because they do. Just as in humans, it can lead to illness.
I hope the antibiotics have him doing better soon!