Husband's midlife crisis around money and success

We are a professional couple with four young children. It started 5 years ago when our eldest was born, he ended up churning through 100K trying to make a business on Amazon. I thought we were finally through it because he seemed more flexible and decided to save and invest in tech funds, with a goal per year + maxing out our super. I'm fine with that. Now he is back at it, wants to start a business and "grow something". Said he's bored with life and wants to be a millionaire. Catastrophising that we won't have enough money to live the lifestyle we want. He's exhausting and I'm sick of these conversations. For reference while I'm on mat lv we make about 300K combined and when I'm 4 days/wk about 400K annually. He won't talk to anyone. Yes I think he's been mildly depressed on/off since the kids but he's extremely determined and proud. No way he will. Any advice please.

-tired wife