I have just read the whole thread and tbh I suspected something like this from the outset.
If you want to support your kids while they are in education, do it. I did. I have a 23 year old graduate at home, working in a temp job earning a bit more than minimum wage, while looking for a job in their field. I've never asked her for a penny. They went to uni away from home and it cost a lot of money to support them, no regrets. They did a foundation degree, followed by a 4 yr degree course, including a placement year, so 23 easily when graduating.
Eldest lived at home during primary degree, worked p/t, never asked for a penny either. Did a postgrad away from home, had a bursary, worked away for a couple of years, back home and paying a nominal amount, saving for a deposit for a house.
Youngest, 19, living at home, just started primary degree, working p/t, min wage, won't be age to contribute same as the other two. Maybe we are mugs, or maybe we just have our kids' best interests at heart. Elder two lived away from home for 4 years each.
I'm worried you've fallen for a guy who only ever wanted a visa... you supported him for YEARS and yet he now has the gall to complain that you want to support your son? WTAF?? You owe this guy NOTHING. He married you to get his visa, knowing your family circumstances, and now he wants to get between you and your children? Tell him to fuck right off! His subsidising your DS is minimal compared to what you have willingly shared with him!!
He may have a point re your DD and your DGS, but set in context, it's up to him to find a way to make WFH work for him, should it mean installing a pod in the garden. What mother would turn her daughter away when she needs her? I do think you DD needs to be supported to help herself more - that's no existence for a young woman. She needs to have independent support, and maybe get a job - did she also go to uni?
Take a step back. What would you advise a friend in your situation? Above all, do NOT let this man control you. I would be very wary that he's been playing a long game - attractive older woman with the means to support him.... who is even prepared to marry him to prevent him being deported... now he's in a job for a short term, and he's trying to dictate terms so far as your children are concerned - red flags galore... I hope it's not the case but looking from the outside in, I have my suspicions.
I hope you have protected your family home, and if you haven't, do whatever you can right now to do that, if possible.