I agree with dave:
1. Be blunt - ISTJs can sometimes be dense when it comes to people being romantically interested in them. So being blunt is best so that he gets the message that want to "kick things up a notch". But, like dave said, be soft spoken as well. Don't be too forward/forceful.
Yes, be blunt, but don't scare him. Forward is a better term. Yes, be forward. In regards to ISTJs being dense in regards to feelings of romantic attraction, I think it is more so that we pick up on it, it's unconfirmed, we aren't sure, we hate to be wrong, our T gets in the way a bit, and were are inhibited by our introvertedness.
2. Be friendly - Yes, it's important to keep conversing with him (flirting with him) like you've been doing. I can't speak for male ISTJs since I'm a female but personally whether or not a person can actually hold good conversations with me, is something that keeps my interest.
Good.
3. Make the first move (i.e. suggest going out sometime over the weekend) - ISTJs can sometimes be shy or, maybe more accurately, ISTJs can be scared to take a risk. So if he's even a little bit in doubt of whether or not you like him (even if you think you're being obvious, an ISTJ may not pick up on it all the way) he may be cautious about asking you out. So if you want to progress things faster, then you should go ahead and take the initiative and suggest something. If he likes you too then he'll be relieved and he'll appreciate the reassurance that you actually want to spend more time with him.
I think this would be best.
Posssibly if not probably. You could even get him to make the first move the question is whether or not he'll see it as a "move." Odds are he would just want to spend time with a friend while maintaining that slight possibility that something might happen between the two of you later on. ISTJs can be rather patient.
Also for the date:
4. If/When you two make concrete plans to get together (i.e. 11:45pm at this spot), try to be on-time and follow through with it. Also try not to make last minute changes to the plan like "oh hey, i'm doing blah blah right now can we just go to a later show?" or "i'm hanging with a big group of friends (read: people you don't know), do you want to just come over and then we can do what we planned next weekend?", etc. ISTJs are relatively emotional stable (IMO) but sometimes if we are really looking forward to something that we planned ahead of time, we can get really angry/put-off if someone else purposely ignores or changes those plans. Or putting an ISTJ in an unexpected situation in order to get him to "loosen-up" might actually make him feel uncomfortable.
So if you want to make a good initial impression, you should be on-time and follow through with the plan

If you want to be "spontaneous" then you could always plan for that. Say something like "okay, let's go to the movies at 7pm then afterwards grab a bite to eat, then walk around the city and
see where the night takes us(unplanned)", or something like that.
In regards to being on time, if he's balanced a few minutes late or not doesn't hurt you but push it past five and it might.
I don't think it's just things that we are looking forward to. I think it's anything that has become finalized in our minds. An example. My mom was recently pregnant with her ninth child. I had resolved myself to another sibling and hence became attached to that fact that there was going to be another one. My mom just this past week miscarried.
Now I'm disappointed and sad.
It's anything that we have resolved ourselves to doing. Now, sometimes the ISTJ might just be going with the flow. That's always fun. 
Inregards to spontinaiety, we love it. A bunch of us find it attractive if it's in good taste. Keeps us on our toes, brightens up are day, is slightly outside the routine, and sometimes is refreshing in a draining sort of way. It's good if it's planned spontinaiety. 
Planned randomness is always fun, but your date or friend taking you by surprise is fun as well.
This thread might also be of use to you:
http://www.typologycentral.com/forums/sj-guardhouse/15372-how-not-pursue-istj-5.html
Key excerpts:
- Don't swear a lot: Yeah, just...don't or try not to.
- Don't be
too friendly or always upbeat: This can sometimes scare ISTJs away.
- Don't be really drunk and neanderthalesque: I'm not sure if this a universal ISTJ turn-off though...two other ISTJs mentioned/agreed with this before and I also agree.
DON'T SWEAR. ISTJs are known for traditional values. In regards to the drunkeness, everything in moderation and good taste. 
Finally, friendliness and upbeat are good things hence why ISTJs like to chase after ESFPs. However, eventually/sometimes things need to be toned down and get serious/thoughtful. It gives us time to recharge and connect. Hence why ISTJs also like going after ISFJs and ENFPs.
Although, I'm not a
male ISTJ...so all of this could be completely off-base

Oh well, I hope this still helps you!