Well think of it like this. His friends (and probably yours too) are likely out partying and drinking and flirting and such, and he's in a dedicated relationship. It doesn't sound like he's a bad boyfriend but it's not the right age to be talking about marriage and kids when you guys don't even have solid plans for how you would afford that marriage or those kids. That scares guys! You want to have kids -- who's gonna pay for them? You? Him? He's young! He doesn't want to think about that now! It doesn't mean he doesn't love you enough though =). If you get to be another year older and he still doesn't want to talk about it and you want to more than ever, then maybe that would be a sign that you need to move on to somebody who's more willing to fit your plan. But who knows, a lot can change in a year and maybe you'll start thinking more like him.
Keep this in mind: The average marriage age is 25 for females, 27 for males. Most people like to get their education, get a career going, and then marry once they're secure enough to afford things. I'm 21 and have been married 3 years. I don't want kids right now b/c we're just not stable enough -- neither does my husband. We married early b/c we knew we wanted to be together. I waited til he was on the same wavelength as I was. Give your man some time too =).