Have you tried clicker training?

Here is the idea:

Get a 'noise maker' like a whistle, or goto your pet store and buy a 'clicker'

Click in between breaths(when he is quite) and treat with a small piece of game - as small as you can treat so he is getting rewarded- just not big so it takes him a long time to eat it.

click and only treat the silence. It's ok if you mess up at th begining, but try really hard to focus, click and treat when he is silent. Do this for 5 min session.

wait 30mins.

another 5 mins session, repeat the clicking and treating his silence.

wait 1 hour

another 5 mins session.

enough for that day.

next day, repeat the following days training.

you should see improvement rather quickly, within a day or so and silence witthin the week.

Just remember to only feed him when he is silent!

g luck,