Changing your pin numbers might seem a bit like a "betrayal" but it absolutely isn't. He can't control himself around money and drugs, he has proven that to you very clearly, no question. He has asked his Mom to take all his money and run it by you. Your money is not safe with him.
Please change those pins, lock down your accounts, if you have a joint account that you use for anything I would get rid of it. He can deposit money in to your account for joint expenses if that is usually the norm.
Now that part does not make you the "money police" you are just protecting your own finances there and that's just common sense at this point. The whole money via Mom via you thing is a different issue. I mean you can try it if you like but how will you settle disagreements?
When you come between an addict and his drugs, look out, you will always be secondary, everyone is, including the addict.
I would also suggest that you immediately set up a payment plan for him to pay you back the £3000. Protect yourself, emotionally and financially, that's really the most important thing for you right now.
Perhaps you can take the day off work tomorrow? If not, all you can do is steel yourself. This might sound silly but practice maybe, in the mirror or just to yourself. Hey how are you, smile and say good, you?
How was your weekend? Not bad at all! By tomorrow you will have your responses set. Another thing to try, when someone says how are you, don't think of him, think of your cat or dog or your favourite movie, whatever brings you happiness.
Keep reading around, the more you know about addiction (for you, not him) the better.
You might also want to check out the F&F of alcoholics forum too, as it's not as quiet as this one, feel free to post over there too, of course, if you like.
https://www.soberrecovery.com/forums...ly-alcoholics/