This situation is unfair on you. And it's a big dilemma of what you'd like, over whether the practicalities can be overcome. On top of this, your ex is free, and you have the responsibility of the children.
Absolutely, your partner can take on the financial responsibility of your childrens' school fees. He can make up the difference he would make to your finances, compared to how it is now.
The thing is, can he afford to?
I have seen this before. You are caught between a rock and a hard place.
It might seem scary, but truly, it is for you to make this decision (not your partner nor ex, nor your children).
I feel for you, so i will tell you my experience, with my children and steps, who were the same age...
-The children really wanted us to join as families (and couldn't AT ALL make mature decisions, their desire was genuine, but naive, and they didn't understand about money or the reality of it not being a constant sleepover party.)
-My now husband was more influenced by his kid's desire for us to join, than I realised. (And our outcome is very good, and we are happy). So perhaps my hubby was naive, and also didn't think through planning financially.
-To make my decision, I wanted to know that for every child and for me, our new set up would be an UPGRADE (ha ha - the term I used at the time) and not a sad "compromise" my friends advised. My new hubby was in for an upgrade, obvs :-)
- I noted that my house sale generated far more equity than my new husband's. I keep this in mind about how his larger income is spread between our blended family and that a) I am worthy of it, we are a couple and also co - parents to both sets of kids, and b) it will take years for him to make up the extra money I put into the house, and c) my ex husband still pays maintenance for my children.
BUT WHAT IF, becuase of money, the only choice is your boy leaves boarding school?
I would say No, don't join forces. Sorry. ðŸ˜
Put education first. You'll be a mum a very long time, and the childrens' childhoods go by so quickly. A school change, and a change at home is a lot to cope with. The step children influence the family culture, as a mum you lose independence, and living together the children have to be treated the same and there are wins and losses. Please note that if you truly believe the Good School is best for your children and you can get bursaries, then this might be worth more to you than the chance to 'move on' and live with your man, which will be (I promise) a complex, difficult, plate spinning emotiotional roller coaster!
There is a good option... If yours both end up boarding, could you 'sleep over' more at your partners? Many men would be satisfied with this arrangement! I really know that this would have been a sacrifice for me, because I longed to live with my partner soo much! But then, hubby was less unhappy with separate houses to be honest. But he did want to live with me, but his longing was intermittently strong and then it'd slacken off. Once married he loved living with me more than I did him! After buying together, the reality was quite stormy for a few years. Step families are tricky and it took 4 years to settle down. Had I felt guilty about trashing my children's education opportunities, I probably would have been broken entirely! But, as on paper I had ensured it was an UPGRADE, I held no such guilt.
I'm sure you are in a real anxious state with this dilemma. I feel your pain. I've seen couples break up in the same circumstances. It's often the women who miss out on moving on.
We sold two houses, bought one big one, blended the children, sent them to the same school, operate a "bio - parent first" parenting scheme, support each other, and we have all benefitted compared to the alternative- remaining separate. It can work. In my situation the ex's have caused trouble. We've spent money to help the 'blending', we have a huge mortgage, but I wouldn't change it because it's lovely! 😊 This (I'm guessing) is what you would like.
I wouldn't have done it though, if I could have got private ed for my children. I would have waited (and maybe lost my partner 😢)