This happened to a friend of mine a few years back, she was working in the hospitality industry. Like your DH, the MD was from a wealthy background and owned the business, family run.
The interesting thing about my friends experience was. The MD was known to behave in a passive aggressive manner to staff he didn't like, and they would then leave. So he effectively used to bully them out of their job.
My friend worked for this man for about 4 years and he used to tell her how wonderful she was at her job, but she saw first hand how he used to treat other people and they would then leave. She admitted to me she ( like all the other staff) never called him out on his bad behaviours to staff as it wasn't directed to her, until it was.
He decided one day he didn't like her anymore and effectively over a period of time started criticising her work, ignoring her, not inviting her to Teams nights out and treating her in a manner to leave her no option but to leave her job. She was deeply hurt and stressed about the whole experience, no one got involved.
So, others saying why doesn't someone else stick up for your DH, experience has taught me, no one really cares if people are being treated unfairly as long as it doesn't impact them. Its very easy for people to say they would not tolerate this, but until you are faced with this situation, you really don't know how you will behave. This was a hard lesson for my friend, but it is true in all walks in life.
I do feel for your DH, it really is a horrible way to be treated. I would suggest you encourage him to find a new job next year and to hell with the lot of them. People can be very cruel.
As soon as your DH leaves the MD will most probably move onto bullying someone else.