I had a boss who was very good and kind but was constantly explaining how mistreated we were by 2 levels up, which was informative at first but then very depressing and frustrating and de-motivating since I couldn't leave the job.   During one long session, I told him a true story, how I had worked in a place where I was made miserable by someone who had one foot out the door who pointed out all the bad things constantly, where beforehand I had been enjoying the job. Eventually that person left for a better job, but I was stuck in my job feeling very bad about it. I told him I have had to learn to isolate myself from people like that, for my own sanity. At some point, a person has to either leave a bad situation or learn to be reasonably positive about it, or at least stop poisoning the environment for everybody else.
   
I didn't think much about it, I just kept some distance and stopped starting casual conversations with the guy which I knew would degenerate into gripe sessions.

Months later, I realized he had started a get-it-together program. He started getting healthier, moonlighting at a new career, polishing up the resume, etc. Coupla years later, he landed the dream job and left for it. I don't know how much my conversation had to do with it, but I expect it did lead to him asking himself some tough questions. I'm sure before I said anything it never occurred to him he was an 'environment poisoner'.

Anyway. As all psych people will tell you, you can't debate the guy into stopping this kind of behavior. But you can telegraph that negativity gets YOU down and bothers YOU, and if he gives a damn, he MAY make an effort to slack up on it.   If he's a total jackass he may decide you're a weakling who has to go, but if he's a good person it may make a change. Be sure you know which it is, if you need the job.