Taking him out on the lead is the first step. This stops Jett being able to carry on rehearsing a behaviour you don’t want. It also breaks the habit of ‘I’m out, I bark!’
The next step is to watch him and get his attention BEFORE he barks. Have him on a longish lead so he can move around fairly normally, with you observing. As soon as you see his head go up, or him turning his attention to a squirrel or a bird overhead, get his attention immediately, before he gets the chance to bark. This is tricky at first as the timing is crucial but it gets easier. Because you have him on the lead you can quickly move in close. Do something fun but incompatiable with barking - we do hand touches, where Henry has to drive his nose into my hand then a treat magically appears in that hand, like a dog treat slot machine! Find something like that, practice it a lot in the house, then use it as a distraction in the garden.
We are still working on this, but I am finding that Henry’s garden barking is much decreased, and that if he does start, it’s easier to get him to stop as he remembers that the magic-hand-of-dog-treats is more fun than barking. I personally don’t think a ‘quiet’ command will ever work for an excitable cocker already ‘over threshold’ in a barking jag.