You have several options.
It really depends on his cycles.
For example, let's say some morning or evening, you test him and you find that the number is lower than 200, or whatever number you would think about skipping the shot. If you DO skip that shot, then the next time you are due to shoot, you can just shoot early. Like if tomorrow morning, he's too low to shoot at 9 am. You can skip, shoot tomorrow night at 7, and then pick up a 7 am and 7 pm schedule. That's an easy way to adjust time.
Or, if you see a number at +10 in a cycle that is high. You can shoot 2 hours early, and then you are where you want to be moving forward.
Those are two ways you can do it.
Another way to do it is to just shoot 1/2 an hour early one day. Then the next day you do the same thing. It would take four days to get to the point where you are shooting at 7 and 7. Of course, that assumes that the numbers are numbers you feel comfortable giving insulin on.