Op just call the mortgage company and say you want off the mortgage, if this means he has to sell them better him being in rental than you. Any equity in the house you’re entitled to Half the profit when it’s sold and the mortgage paid off.
If he remortgages, you need a legal document stating you stay on the deeds and still own fifty percent. So he can’t do you out the profit.
Also if there is equity you can force a sale and use the money to buy a better home for yourself or use it as savings.
You have a limit, everyone does, that they can borrow. Doesn’t matter how many mortgages it’s spread over. So if on 14k the most you can borrow, for example is 50 k, but you are already on a mortgage with that owing, then you’ve used the lot. The total you can borrow is inc the other mortgage,
Again it doesn’t matter what “reasons” he gives. You’re a grown up and need to take personal responsibility. You can choose to stay on the mortgage preferring him to have an owned property and you in rental. But that’s a choice you’re making.
Do you really think he’d do it for you? Because he clearly wouldn’t.