I agree with Arbiter Dewey...you should wait. If you leave him now he'll likely think that you were too bummed out/scorned to stick around, and then will cut his losses and tell himself that it wasn't meant to be anyway. In my experience, ISTJ's have very little confidence when it comes to dating...and I think this was said earlier...we are not optimists in this arena. It takes someone special to break down our walls and prove to us that they are worth "the trouble." I think your chances are better if you stay.
Of course I say this with the intention that you don't lose your self-respect. Don't let him and his indecisiveness steamroller over you. There comes a certain point where you do just have to let them go and hope they'll come back if it's meant to be.