I don't think it's a problem at all. I think you could go right back to 9 am in the morning. To put your mind at ease, what you can do is test him at 8, then again at 9. If the number is about the same, or rising at 9 am, then I think giving the shot then is okay.
Just an FYI for anyone using Prozinc. The maker of the insulin, on their website, says that the duration is "between 10 and 14 hours". We say "12 hours" because that's right in the middle of its advertised duration range. You definitely have a bit of leeway in terms of how far apart the shots can be. A good way to tell if the insulin is wearing off is to test an hour or so before the normal shot time, and then again at shot time. If it's rising, and no food has been eaten in the past couple of hours, it's rising because the previous dose is wearing off. As long as the preshot number is a number that you feel comfortable giving a shot on, then it should be okay to give the shot.