The Future of Geoff

My big brother, Ucre is giving me the what-for about Geoff.  Some of what he's saying is inaccurate because of Geoff's personality.  There are some things that are just not Geoff.  For instance:   
"Little says:
yes, but heres the reality of it....men dont change unless they truly see the benifit in it or them selves...and its there idea....if a woman thinks a man needs to change....forget it....the game will be played just as long as he feels that he can still get something that he wants...ultimately he wont change."
That statement is simply erroneous.  I know Geoff.  He doesn't have a conspiratorial bone in his body.  I don't doubt that maybe he's still interested in me mostly for the sex.  That's definitely a possibility.  But believe me, if I know Geoff (and I KNOW Geoff) it's subconscious.
However, that leaves me at an impasse.  I can't allow things to continue the way they have been cause #1, it's not fair to me, or to him, and #2 he's never gonna grow up!  I'm shooting myself in the foot here!  I know I have to do something.  There's going to be one big event that determines the course of our relationship, but I don't know what it is, or how I'm going to make sure it ends the way I want it to.
The thing about this situation is, he's not actually detrimental to my health in any way.  Sure, he's a little immature, and he obviously needs to grow into himself before he can fully concentrate on a significant other, but he's not abusive, he's not a punk, or a shithead, or any other kind of devolved individual.  So if I were to simply write him off, I know it would come back to haunt me.  I am so positive that in four or five years, we are going to be perfect for one another, just as we were when we first started going out.  Obviously things cannot continue in the present vein.  I'm tortured.  I'm so close to him, and yet just that much farther away because of it.  I am pretty sure that what we are doing now is going to be detrimental to our relationship in the long run, but I don't quite know how to fix that so that what I most desire will be the outcome.
I don't even know.  I'm running in circles with this post.
I don't even know if he still loves me or not.  Sure he says he does, but in every single way he is still such a teenager.  His feelings could change at any time.
And the pathetic thing is, even if some sexy young thing burst out of the woodwork right now, I don't think I could move on!  Geoff would be so perfect to love, and marry, and build a home with.  I don't want to move on, and then come back to him later on and have that long hiatus between us.  I will always love him.  So if I choose to, I can wait for him, but I have no clue how I'm going to stay sane in that time, especially if he goes and does what I can't, starts dating someone else.
He says he still loves me as a girlfriend, and he certainly acts like it.  He's still just as protective, just as possessive.  We still hang out, we still fight, we still make out, and make love.  There is absolutely NOTHING different about our relationship except for the title, which is the one thing I need to stay the same.  I'm stupid, I do know what I need to do.  I need to give him an ultimatum: either I am, or I am not.  And it'll be a learning experience for him, giving him more maturity, which is my main objective.  But what if I am not?  I actually cannot let go, especially if he stays a part of the Collective, which of course he will.  I have no right to say who my friends hang out with.  But as long as we have contact, I won't be able to let go.  And it is equally unfair for me to have to exile myself from my friends because of my relationship with Geoff. 
So it comes down to one thing: I need to be with Geoff.  Which comes down to another thing: he needs to grow up.  Which comes down to another thing: I can't make him grow up.  Which should point me in the direction of complete write-off, but then there's the problem of unsavory self-imposed exile.  Seriously, WHAT THE FUCK????
it's so frustrating.