Hi M2T. I do feel for both of you. This problem is extremely common. I have heard that money causes more relationship problems than any other issue.
I really do understand how worrying it is to be landed with all the responsibility for balancing the books, it can seem as if your dh has the understanding of a child but a child that you cannot prevent from being allowed to have access to the money. All the same I think you are being very hard on him because I don't think he can help being the way he is, as others have said, there are all sorts of complicated reasons why he has spent this money, and they are more to do with control, power and feeling valued than the actual items purchased. I know that isn't much help when you find half your savings have disappeared and it's understandable that you are angry.
He's crap with money and he knows it. He knows he needs you to organise that side of his life for him, but in turn that makes him feel like a child and that makes him angry. He knows he has no right to be angry with you, he knows his actions have caused this, but he is still angry. My guess is that he spends because he is angry. The spending makes him feel more of a man, he has notes in his pocket like the next man, he can stand his round in the pub like the next man. He worked hard today, he deserves a little treat, why shouldn't he go to the pub and buy a few drinks? (I'm guessing this is how he is thinking).
People who do this very rarely ever think about the big picture, they think about £2 for a pint today not being a big deal, they don't think about £2 for a pint every day being £700 a year. I would say that this type of person is typically generous and kind-hearted, and very popular with friends since he has shallow pockets and long arms. All this is no help to you though.
I agree that counselling would be helpful since the issue is not really money, but the feeling that money gives him. I also think that he would get a kick out of learning to save, if you could teach him. If you don't feel you can entrust him with any of the financial dealings of your household, perhaps you could ask him to take over things like searching for a cheap holiday on the internet, to see how much money he can save you, or handing over £50 and asking him to do the shop for a week and see how well he can make that money stretch. He needs to know that he is absolutely invaluable to the running of your family. If he's not good with figures, so what?
But I would also stress that if there are any more secret debts he really needs to confess now. Offer him a once-only amnesty and be prepared to mean it. Make sure he realises this can never happen again. Tell him how much money you expect to have for personal spending and ask him if he thinks he should have more. How could he possibly say yes? If you both have the same amount, set, per week to spend on yourself, perhaps it will bring it home to him that you are not somehow richer than he because you are earning more.
I am lucky that both my dh and I are good with money and so we don't really need to worry about what we spend. It's lovely. But both of us have had partners in the past who were bad with it and it is very easy to end up feeling like you have another child to support in these circumstances. I would tell dh that you hate feeling like that about him, he is your love and your partner in life, what can you do to help him feel as indispensible as you know him to be?
My guess is that there is a big part of him that's relieved as anything that you have found out about the spending. He knows now that you will control what he could not.
Unfortunately, I think if I were in your shoes, I would also have to say, once and for all, that I did not intend to work all my life and end up with nothing. If it looked like that would happen, I would leave. But if he would like to get to grips with money and learn to turn it to his advantage, I would support him all the way.
Good luck. It's not easy at all, I know.