wow- what a shame, for all of you.
If you are not going to leave him then if I were you I would be super organised and pretty petty about it all.
I would, get all our finances on paper, what comes in/out what gets spent on what. I would make him a big copie, stick it on the fridge or something. Try and talk about the unfairness of his spending, if he doesn't want to know, then I would do the following.
Budget for him- if he wants to spend £200/300 a month, then he has to do without luxuries, so I would do a bog shop, and normal stuff for me and the kids, the basic, cheapest of the cheap for him, no meat- too expensive, crap white value bread, no beer, no snacks- too expensive.
Make sure all of the bills are paid, then take all the cash out of the account, and give him his £200, then Meal plan- ordinary nice stuff for you and the kids, cheap crap for him. Horrid shower gel, cheap shampoo, scratchy loo roll. No petrol in his car if you run separate cars.
If he is taking out so much money the he feels the shortfall, not you. If he complains, take him by the hand, and have a look at the lovely big spending break down, let him know how it is all in his hands to change things.
Do it for a month, no change, then kick him out.