Tl;dr: Maybe you're communicating two different ideas past each other.

It sounds like he wants to splurge on this knowing it's not the best idea. Like a big expensive purchase, just 'cause. Especially since you say he understands the numbers and everything. So it's not about the facts, just the feelings.

Maybe he's asking you before he does it because the alternative is just buying everything and that would be bad. So he gives you the veto, and that's good! But you want him to decide on his own not to do it because it's not a great idea. If so, you'd be coming at this two different ways, addressing two different sets of needs.

I would recommend that you explicitly state that you don't want to have to veto this (if you haven't). That you want him to make the most logical (optimal, safe, etc.) decision. That you feel like his mom if you have to say no. And just reassess your values and your plans for the future.