Wait, there's something here the others aren't seeing clearly. A seven-year gap isn't the same as waiting two or three. By 35, you two would have a completely different life from what you have now: maybe you'd have changed jobs, cities, priorities. The risk is that by then you won't want the same things anymore, and it's legitimate that this worries you. But before any ultimatum, you really need to understand what's going on in his head.
Because the issue here isn't whether he's right about financial stability (which does matter to some people), but whether that's really the barrier or whether he's using it as an excuse for something deeper: fear of commitment, need for control, insecurity. Ask him what would happen if he suddenly made a ton of money tomorrow - would he marry you then? If the answer is yeah, okay, fine, it's just money. But if he keeps finding new reasons, then you know the problem is something else.
It's not about being tough or threatening either. It's about telling him straight: "I want to marry you, but I can't wait until I'm 35 without knowing this is real for you." If you two can't reach some middle ground (get married at 31, for example, or talk again in two years when things are different), then you do have a compatibility problem. But that's important information you need now, not seven years from now.