So your TDD was around 6.5 to 8 and now it’s lower than that? What is it now?
The obvious quick and dirty culprits if he’s both spiking up high and then crashing down low are your meal dose and the insulin duration of action. It could be the insulin is working slower than usual, in which case, the same dose but prebolused up front more would help.
Another option, which you may already be doing is to give the same dose of insulin, maybe 5 minutes earlier, but immediately set a zero temp as soon as he eats.
Also, it’s possible all the settings have changed.
To check ISF. Wait for a time he’s a little high (say, 150-180) with no food on board, and give him a little bump down (maybe 0.2?). Does he wind up where his ISF would predict? For instance, if his ISF were 200, and you gave him .2, that should drop him from 180 to 140 mg/dL. If he’s dropping to 100, that means his ISF is too strong and you need to make it a higher number.
To check carbF: Give him something with a handful of carbs (say 4g) when he’s at 100 or 120 and not many other macronturients. For us we like a few saltines. Let him rise. See what he peaks at, typically 60 to 90 minutes later. Let’s say he goes to 180 from 4 grams of carbs, starting at 120. That means each gram of carbs raises his BG (180-100)/4 = 20 mg/dL. Now look at your ISF. If 1 unit of insulin drops him 200 points in BG, then that means each gram of carb, which raises him 20 mg/dL, requires 0.1 units of insulin. So his carbF would be 10 grams carbs to 1 unit insulin.
To check basals: you know the drill on this one but it’s a PITA. Just cobble together an hour here or there when it’s been 4 or 5 hours since his last bolus and see whether his basal rate keeps him steady. Honestly for us basal testing is basically impossible but you can basal test specific very narrow time periods (an hour or two) without too much inconvenience. The plus is you’re only looking at time mostly during the day, and mostly around meal times, so you probably don’t have to do this testing for every time of day – maybe just 6 hours total.
Does he have very scheduled mealtimes? Maybe that would help in debugging – try shifting his meals 1 hour forward or back and see how that changes the dynamics of the post-meal spike. For us our son is on a pretty rigid schedule during the week versus the weekend, which makes it a little easier to debug.
Anyways, good luck. Honestly we’ve had little luck figuring out the cause of these super high spikes and sudden drops. My impression is it’s the site, but I have no way to prove it, and our strategy is to just ride it out and treat lows much more quickly on the downward side (maybe even at 150 or 160 if you are seeing crazy drops that day). But we have plenty of roller coaster days that look JUST LIKE THAT!!