The money he spends on his kids is not the issue. You said in the original post that 55% of his take home income goes to them. If this is TRUE after tax income then I don't think 55% is too much to pay to put your three kids through college. If he was still married wouldn't 55% of his take home pay go to providing for his three kids?
I get that OP and her spouse are high earners, so the math may be different, but yikes. By your math, that's 55% for three of his four kids, which would mean 73% for all four kids. Even if we attribute half of theoretical family spending on necessities (housing, food, clothing, healthcare, etc) to the kids (either direct payments or as part of child support), that's way, way too high. That's 27% to pay for the other half of necessities, contribute to retirement savings, and for the adults to have any spending money.
Put differently, it means that, assuming ALL of the rest of the income goes to necessities, then 45% of the family income is going to non-necessary spending on the kids, while the adults get zero non-necessary spending, including retirement savings.
Even putting 55% towards all four kids combined would be a high number, in my opinion, particularly if it doesn't include expenses like daycare (I don't think of daycare as spending 'for my kids' so much as spending 'for my career,' but that's a separate issue).
As another data point, the calculation of expected family contributions for college maxes out at a 47% *marginal* rate (the effective rate would be lower, perhaps substantially so, depending on household size and income).
In this situation, OP and her husband should absolutely be downsizing, and he should make all of his court-ordered child support payments. But she is not being unreasonably to expect him to discuss ways to cut down other spending on his kids, because he simply cannot afford it.
For OP, I think one helpful basis for this conversation would be to run the numbers on how your finances would look if you did get divorced. Assume that you'd split the custody of your shared child 50-50, and include state standard child support based on that time split. Not because you should do that, but to put a number on how much you are subsidizing him each year.
And to be clear, spouses subsidize each other all the time. I paid off my husband's student loan debts for him after we got married, and have zero resentment about that! But that's because we run our finances as a team, and make decisions together about what will provide the optimal outcome for our family. Your husband wants to have it both ways. He wants shared finances for the purposes of having you subsidize his lifestyle, but he doesn't want shared finances for the purpose of financial decision-making. He can't reasonably have both of those things at once.