I think this is the issue - he absolutely should NOT hand in his notice until he has another job lined up, especially in this market. Depending on the age of your DC you could also start planning a return to work, even if it’s just part-time. You may have agreed that this was the way you wanted things years ago, before children, but things change, people change, work environments change. He economy has completely changed in the last 5 years. It’s not the same world as the one in which you made that original decision together that you would be a SAHM. You need to be able to review.
It sounds as though he is deeply stressed and feeling overwhelmed just now, so the thing to do is calm him down and make a plan before he does anything. Sit and work on his CV together this weekend, contact some headhunters/recruitment consultants (I assume at his level that this is how it works?). Get the ball rolling and at the same time, you brush up your CV and start looking at what jobs you might be able to do PT to segue back into work. Or can you explore retraining?
Often men who are the sole breadwinners actually feel especially burdened because they KNOW they if they lose their jobs, the family has no safety net. It’s an added pressure during difficult times to know that everyone is dependent upon you and that you are, in effect, trapped whether you like the job or not. Despite it being bloody hard work, being solely responsible for children in order to facilitate your DH’d career, it is - nonetheless - a privilege that his income has been able to afford you and his children. Many women do what you [and I, a fellow SAHM] do whilst juggling FT jobs and elderly/sick parents.
When you criticise his lack of involvement, you need to remember this is what you both agreed: his sphere of operation is work/funding the family; yours is the domestic environment. You can’t moan that he is not engaged when he is doing long hours and carrying 100% of the financial burden unless you are prepared to take on some of that in exchange for his doing more at home. And that is an okay discussion to have. It may make him feel less stressed if he knows you are planning to return to work/retrain once both kids reach a certain age; it may make you feel less vulnerable to know that you do have that second smaller PT income that, perhaps, you could start putting into savings?
My DH is in finance - the events of the last few years, globally and domestically, have made his job 10x more stressful that he’d anticipated. Financial regulations seem to be changing every bloody day (only to be U-turned upon several weeks later) so many companies have no idea which way is up.
I would, as many have suggested, NOT respond by ‘WTF, if you do this I am divorcing’ but by asking him to talk though what has triggered this scenario and discuss whether it would actually be any different in another company right now, given the political uncertainty. I’d ask whether perhaps arranging some counselling via your GP (you can actually usually self-refer so don’t even need to see a GP) so that he can have a safe space in which to download and decompress, because if you implode every time he loses it - and fan the flames of his stress and angst - that doesn’t help him. He needs a space where he can speak freely without fear of triggering you [and a divorce ultimatum - I mean, in his shoes, how does that look? As though you only want him in your life because of the money he brings home. That would make any human being feel like shit.] And then ask him what could change at home and in your marriage to help him cope with work, and perhaps explain that you also feel unappreciated and might like more support at weekends etc. You need to communicate and take this opportunity to renegotiate your marriage and relationship - not throw ultimatums at each other.
If once he’s explored counselling and job opportunities you still think the relationship isn’t working, then that is another separate discussion 6-12m down the line. Right now, you need to keep him in his job and focused on fixing the issue/finding a new one and then take the next step.