So, we found that no-carb foods spiked our kid to 300 whereas just skipping a meal had no such effect ....so the no-carb meal doesn't necessarily work. Honestly, just not eating is the best way. If he's in range and it's been 4 to 5 hours since his last insulin dose, just let him drink water, crystal light, eat sugar-free jello-- things with no calories -- and watch what his blood sugar does as long as he will tolerate not eating. So let's say you bolused him for breakfast at 5am and now it's 10am and he's vaguely in range (say, less than 200) and hasn't had any bolus insulin since 5am, maybe just start watching his BG, try to hold off on lunch until 1pm. If he sleeps in the car, you can take him on a long drive while you just watch his Bg. Or let him watch a movie he's super engrossed in, or take a long nap -- whatever it is, just so you can hold off eating for those two to three hours. Then you just watch what happens with his BG with no food and no insulin in his system. Does it rise more than 30 points over that 2-3 hours? If so, basal is too low. Just rinse and repeat for all the portions of the day. If it's dinner you're basal testing, just serve lunch at 10:30 or 11am and then don't eat until 8pm, that should give you a basal rate from 5pm to 8pm. If it's breakfast, just let everyone sleep in, watching cartoons, and then go out for a late breakfast or brunch around 10 or 11. Nighttime is obviously the easiest to basal test for as he sleeps. There will be gaps in your hour-by-hour basal test, but it's better than nothing.
Also, I second what Lorraine says. Be bold with the insulin! If you're in the 300s, you have a long way to go. One way I think about it is this: if your I:C ratio is 1:20, he starts off at 100 mg/dL, eats 10 grams of carbs, and at the end of 3 hours is at 200, that means you have 100 points of BG that were not addressed by the food insulin. Then look at your ISF -- let's say it's 1:200. That means he should have gotten an additional 0.5 units of insulin to bring that high BG down upfront. Obviously, insulin doesn't work quite that way, but it's a good starting point. So if his original I:C ratio recommended 0.5 units and he really should have gotten 1 unit, then that would suggest your I:C ratio is actually 1:10.... Obviously this is not exact and you shouldn't go up all at once by that full amount as insulin resistance sets in once you've reached the higher BGs, meaning you probably need less up front. But the point is, if the change you're making is 1/5th or 1/3rd of what you'd ordinarily be giving to get him back in range, then it's unlikely to make a huge difference.