Normally you do it 1h before he wakes. Since he's got a range of NW times, I would go by the earliest one you mentioned and give it 3-5 nights to work. 3 nights is the earliest I've typically seen it work, so hang in there
I'm not sure that you can extend his wake-up time if it's the time he typically would wake after a good night's rest. I've not tried it for that purpose (have for a habitual NWing), so hopefully some others will chime in...
If you're looking to push back bedtime, doing it in ~15 minute increments (perhaps more if he's older?) can help. Again, hopefully others will chime in, too