If there's any chance you or he want biological children, this is a crazy path to go down.

First, normal people don't get vasectomies with the plan of having children later. A vasectomy is a permanent sterilization procedure. It's entirely possible (probable) that he doesn't want children at all and just isn't saying that.

Second, even if you two both do want children in the future, but for some crazy reason aren't considering reversible birth control, banking on the much reduced (versus typical fertility for a healthy couple with no fertility issues) chance of pregnancy if you put together the tens to hundreds of thousands of dollars you might spend on reversal and/or other assisted reproduction is just totally irrational.

If you're legitimately worried about the cost of having a baby but want one in the future, it would make more sense to not intentionally put yourself in a position of needing expensive care to become pregnant. If money was legitimately the concern, wouldn't it make more sense to put that money away in savings to plan for a child?

And if you or he honestly want (or even might want) a child in the future, can you really comprehend how utterly devastating it would be if your plans fall apart? What if you never have enough money for these expensive procedures upfront and raising a kid, or you can't cope with the physical and emotional trauma of repeated fertility treatments, or you try your best and just can't have a child at that time?

No sane, ethical doctor would tell you to go ahead with a vasectomy with any kind of future plan to reverse it. He'd have to actively lie for a doctor to perform this procedure on him.

You two need to have a serious conversation about if children are ever on the table for either of you and act accordingly. Him having a vasectomy is just delaying that conversation until it might be too late for either of you, with consequences that can be permanently emotionally devastating.