Welcome to the site.
Well, it seems like your husband's professional life which has been always stressful as you had mentioned, is now taking a toll on his health in a way that he is stressed out and is seemingly slipping into depression as is evident from the recent observations that you have made about his character and attitude which has changed from being a positive and happy person to the presently seen negative and sullen individual who is loosing his grip on his calm and composed persona.
So it is my suggestion that you get him to see a clinical psychologist who shall first assess his symptoms and thereafter who shall start him on an appropriate specialized therapy such as CBT ( Cognitive Behavior Therapy) which will help him to unburden himself off the stress that has mounted on his back and also he will learn some core stress coping skills which he shall make use of in his high powered job and demanding lifestyle so as to stay afloat with his sweet disposition being intact at all times.
Other than the therapy you may ask your family physician to place him on an anti anxiety medication such as escitalopram ( lexapro ) or/ and buspirone ( buspar ) for few weeks so that he can overcome stress and calm down. Once this is done then the drug can be brought off. If the family doctor cannot prescribe this then he can refer him to a psychiatrist.
So basically your husband needs professional help in the form of therapy from a clinical psychologist alongside medication and if this is delayed any further then he may slip into depression which will require more rigorous treatment plan to counter it.
I hope this makes sense and gives you some sort of perspective about what you are dealing with.
Wish your husband a speedy recovery to his normal happy go lucky self.
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