5 years is plenty of time to decide whether or not you want to marry someone. There's nothing you need to do when you're single that you can't accomplish while you're married. A person doesn't need to wait until they've finished school, gotten a certain job, or gotten to a certain financial position before they get married. All of those can be accomplished while you're married, and frankly marriage usually makes it easier because then you're a team and can support each other. A good marriage allows room for both partners to grow, explore, and achieve.
The only "valid" reason why he wouldn't want to get married yet is that he doesn't feel ready to make that kind of commitment. And if that's the case, the decision is in your hands. What's more important to you? Being with him or being married? There is no right or wrong answer to that.
Good luck!