The main argument for injecting before a meal is that if you give the injection at the same time as you eat, the carbohydrates in the meal start to be digested after about 15 minutes, but the insulin will take longer than that to act. So you get a blood glucose spike after the meal.
It depends on a lot of things, if my pre meal blood sugar is very low then I do sometimes eat the meal, and then inject, sometimes I have had a hypo just before a meal, and started eating the meal, but injected later.
It depends what the food is, how spread out, in terms of time, the meal is, and lots of things.
If you think about when the bulk of the carbohydrates in the meal will be eaten, then injecting about 20 minutes before then would probably be best.
If your meals are nice long sociable events, then there may be a case for waiting. But I wouldn't wait just because he doesn't know how many carbohydrates he is going to eat, he can try to assess that before the meal, at least to some extent, and always correct a bit afterwards, if he got it wrong.
The thing with diabetes is people have to self manage quite a complex system, and there are so many variables to think about, but if he is getting a spike after eating, then looking at the timing of the injection might be a good thing.
He could just inject a couple of units before the meal, and then the rest later on, though in doing that, he will be insulin stacking, and may need slightly less insulin as as result, because two injections close together can turbo charge the second one a bit.
Trying to avoid spikes if he can is a good idea, but the best way to do it is a complicated thing.
