I think that for you will need to start another account that he doesn’t know about. This could be done in another bank or the same. Do not transfer the money directly over as he will see it in your bank statement. When you get cash out get extra and put that into then he wont know.
I did explain but mine was slightly different as we had joint accounts so they sorted he being took off it.
Hi open with different bank , ask for no documents to be posted and never show money been transferred from that account in or out , memorise bank sort code and account no and leave card with somebody else
Do you have access to the bank account to get food shopping? I used to get a bit extra cash back when I did the food shopping and it didn’t show on my statement. It would just show a payment to tescos. That way I could get a little cash without him knowing. You may be able to pay this into a new account that he doesn’t know about. It could be time consuming to get much money paid in but it is best to act carefully so that he doesn’t get suspicious. Good luck and stay safe.