It sounds like he's something really special to you, given the fact that he's making you "completely CRAZY", which as an ISTP I'm sure is a state of mind that is highly foreign to you. The voicemails, the extreme sensitivity to his thoughts/feelings, the slight obsessiveness ... none of that really sounds like typical ISTP behavior, even in a relationship. You guys are usually cool as a cucumber. Of course you are a female ISTP, and you've had sex with him, so gotta take that into consideration...
Your feelings for him must be overwhelming you, just as you say, and usually when T's encounter feelings, they try their best to shove them under a rug until they feel they've gotten it under control. And you must be feeling so OUT of control that I am actually surprised you haven't tried to break it off with him. I think you putting those disclaimers out there is basically you setting boundaries, so you know what to expect from him, even if it is nothing. As long as you "don't expect" anything from him, he can't hurt you... you'll have second-guessed that potential out of him.
He senses that he's driving you crazy at a very transitional point in your life, and because he cares for you, he wants to do whatever it takes to make sure you are on the right path for yourself and not overly distracted. INFPs are very loyal and devoted, so if he genuinely likes you, he'll wait and stick around until things are smoother sailing for a relationship.
INFPs and ISTPs do have great chemistry, and get along quite well together whether it's in bed, getting each other's humor, being attentive, appreciating each other's space/alone time, or enjoying new experiences together. No wonder you're crazy about him! I'm happy to hear that you are sharing how you feel about him TO him. INFPs pretty much can't get enough of verbal affirmations (or even just anything feeling-related), from somebody they truly like back.
If they're not as into the other person, then excessive doting and mushy feeling-talk will make them feel smothered. My hunch is he doesn't feel smothered by you--he probably thinks your gushy voicemails to be very cute and endearing. Especially if he really cares about you AND he knows you're not usually like that at all. It could make the relationship all the more special to him.
Anyway, I don't think you have anything to worry about; it sounds like things are gong pretty well! Hope this helps illuminate things a little!