My take on this has changed since you said you have adult children of your own. They are through their education (unless you're funding uni fees.)
If you're living together and have a joint mortgage, you are in effect in the same position financially as being married (I suspect you're the higher earner.)
I assumed from your earlier posts that you were childless and part of your reluctance to pay these fees was you may have children with this man in the future. And that you wanted to ring-fence your income for those extra outgoings.
Your argument of 'we could split up and I'd lose that money'- would you say the same if you were married and a step parent?
One option would be a loan, a legal contract that your partner would repay.
The same surely as the arrangement over the finances if you split up now - you're both paying the mortgage, but aren't married, so presumably you have a Deed of Trust ? That way you each get back the percentage you have paid for the house.
My advice is you think very seriously over what you want from a relationship where there are existing children- yours and his.
At what point do you wash your hands of expenses for children who are not yours? This is fundamental to any blended family.
You're either in it as a couple (IMO) or you decide to have red lines between what you each contribute not just now, but years ahead.