Well, if he doesn't want to do it, it won't work. What about asking him how he'd like to manage it? I have two rules for when I disagree with DH on something:
1. You can't choose an option that puts all of the burden on the other person; and
2. You can't just veto an option without proposing something yourself.
It sounds like your DH is perfectly happy with the current system, because you do all the work and he gets to go buy whatever he wants. So if you have both decided that you need to cut back on "extra" purchases to support another goal, how is that decision going to be enforced? One way is an allowance; another way is to mutually discuss and agree on every single purchase before it is made; another is to use cash-only to make sure the limit is not exceeded; etc. And of course there are the non-desirable outcomes, like him continuing to buy stupid shit and you turning into nag/enforcer/mom; or having no enforcement mechanism and wondering every month why you're not making progress toward your goal.
You may actually have to start with some of the "bad" options before he will see the value in the others -- we moved to the allowance system after originally planning to combine everything, because I could see myself turning into a shrew over DH's stupid spending. I think it is completely fair for you to say that what you are currently doing isn't working; that if you guys are really serious about your goals, you need some way to enforce that day to day; that you thought the allowance system was one way to get there; but that you understand he doesn't find that particularly appealing, and so you are very interested in hearing his ideas for how to address the situation.
And if he then says what my DH did in one particular conversation ("that's ok, I'm perfectly happy with the way things are right now" [where I did everything]), you can respond like I did ("Haha! That's funny! No, really, what's your preference?").