As a practical suggestion, you might try both using a joint budget tracker for a couple of months. The kind of system where you have to track and record every single purchase. It can be a pain to do it every day, but a) it will demonstrate to both of you exactly where your money is going, and b) the simple act of knowing he has to record every purchase and share it with his partner might help curb his impulse buying. (Which, to give him the benefit of the doubt, might be more like an addiction than him just being thoughtless or selfish.)
You can either use specialist software like YNAB ("You Need A Budget") -- which personally I found a bit too confusing but other people I know swear by it. Or if your excel skills are up to it, just make a shared google doc spreadsheet. Make columns for categories (i.e., 'food', 'baby things', 'rent', 'car things', 'snacks and eating out',) and rows for each day of the month. Have a page each for each month (so you each track your spending, but it's visible to each of you). Make sure you're categories are clear on what are 'family' purchases and what are 'personal' purchases. So groceries you buy that everyone eats, versus snacks and coffees you might get when you are out by yourself; stuff like cleaning products that are for the household, versus his ebay crap which is for only him.
My DH and I did this when we were really broke and having lots of arguments about money. It helped because once it's all laid out in a spreadsheet, it's easier to point at the problem (and admit that there is a problem). For us, it was realizing we each spent way more than we would ever have realized on eating out, both together and individually. With you it might be that, if he is spending ten times as much money as you on groceries because when he goes to buy one thing he comes home with ten others, then that's what you can focus on as the problem, and then discuss (together) a way to resolve.
Anyway, it might be something to propose as an idea (along with perhaps debt-counseling) to try for a few months, to see if he can commit to confronting his problems with money and working to change them.