I am bi-polar, although I don't go full blown manic too often. Right now I am stable and doing well and have a little perspective on this sort of behavior.
I grew up in a home with a Dad like you are talking about and it isn't fun. How is he with your kids? Does he take any "moods" out on them for instance or even you?
He is engaging in some destructive behavior and unfortunately only HE can change it. Somethings I would wonder is:
1. Is he aware he does this or is different from other people or is this just how he is and he is oblivious?
2. Was he ever different than this?
3. If he is aware of it, does he enjoy being like this? Ask him.
4. Does he want his children to grow up and live in this same pattern?
His children (maybe not all of them) will grow up and behave this way. I grew up with a Mother who was all light and joy, but who am I more like- yeah, Dad the pessimist- so is my brother and there were only 2 of us. It is so learned that I still struggle each day not to be that way. I am like a grown-up Mrs. Eeyore. My glass is not half empty- it fell onto the floor, broke, and made a huge mess!
My hubby and I freely talk about this behavior and so he is quick to point out when my thoughts or speech patterns seem to be taking a bad turn. While irritating, it does help me to right myself and see how ridiculous I sound. In my head it doesn't, but when he says it, it does.
Counseling would be great, but I think if he will start even talking with you about this and you could open some dialogue you can get far. His family wouldn't have to know about his going to counseling. I am assuming they are just anti-psych?
Does he have triggers? Find them out (like talking with his Mom) and find out what about that is triggering him. Is she saying something negative about him or just to him? If she is just going on like gloom and doom he needs to learn how to let that sail in one ear and out the other. He cannot take on her junk obviously when he has enough to deal with.
Would he possibly journal? Men hate this, but if he could spend some time writing about how he is feeling it might help a pattern emerge.
Write down what he says and then write out a positive replacement he could have. Make copies and put them where he will see them daily. I did this and had them by my bathroom sink.
I agree with previous suggestions about self-talk. He needs to learn a whole new language there. It will take time.
If all this falls on deaf ears, what happens if you do fun things without him? Does he get mad or maybe would this spark some interest in going? Every person is different.
To keep your sanity, counseling for you may become necessary and may help a lot!
I wish you the best, I know its rough!