Is he stringing me along?

You seem pretty serious for a girl of your age. Personally I don't see the big deal in being married. I mean you have a lifetime ahead of you. I think if he's seeing you exclusively and the two of you are living like husband and wife then whats the problem? You may be thinking of having kids but I can tell you now that its hard work and I would advise any girl or guy to experience everything you can before going down that road cause you will be tied down for the next few years for sure and then spend years more worrying about them. Don't be rushing things and just enjoy your youth if you can appreciate it.