I wouldn't give the shot if his BG is under 150. If it's over 150 & you get a rising # from 2 tests 10-20 mins apart, I would give maybe a half dose? That's probably enough lowered that you wouldn't need to be worried. Of course if you are going to spend the entire day at work freaked out that you gave any insulin then yeah, skip it. ;-)
But I'd say even a small dose is better than nothing, especially if the following shot will be delayed. I've even shot 0.2u before just to try to keep the #s flattish rather than going skyhigh (not really enough insulin to be useful for Bix at least, but still I've tried it, lol).
You can do the same thing with the +18 if the following shot will be early - just give maybe 0.5u or something to try to keep things from getting totally off track, and then give a small dose that's early the next morning, and then try to get things back to normal that night.
Not sure if I totally understood the timing, but try not to stress about it too much, just give what you think will be safe, accept that the #s won't be ideal for a couple days, and then regroup when you can get back on schedule. As long as he is doing reasonably well and ketones are not present, there is some leeway.
Or would it work maybe for Hillary to come over at your normal shot time & test & shoot for you, or something like that?