Because of their aspect of loyalty and trust, they tend to be very literal with their promises, even to a point of being completely silly. But that's because we ISTJs tend to build our world on rules and agreements. As we are IJ people, we are natural control freaks and easily become terrified when things become irregular, inpredictable and undependable. By fully committing ourselves to whatever we hold dear in our lives, we try to stabilize our world so that we would always have the most important things with us should things go bad. This is one of the reason why casual relationships do not suit us ISTJs well.
However, ISTJs who have embraced their Se side more can actually enjoy casual relationships without much commitment. This may be because they are readier to react at unexpected things and do not feel need to safeguard their lives as much as less Se ISTJs.
However, ISTJs are also ready to invest in to their future and are very stubborn on not giving up easily.
Best case scenario: their commitment will hold true, no matter what.
Worst case scenario: they become clingy and refuse to let it go, even to a point where emotional (and sometimes even physical) pain is evident.
But one of the sins of an ISTJ is passiveness. The seem to be prone passing opportunities over taking risks.
I hope you are happy with these answers... and now I have a question for you: Who is he, and why are you afraid of a serious relationship with him?