The actions of the gliptin range of drugs, are to provide more insulin for the first phase insulin response during intake of food. This is supposed to help with patients with insulin resistance or a weak first phase.
And reducing the dosage of insulin is necessary to try and reduce the amount of circulating insulin that is not used.
This I have seen before when metformin has awful side effects on someone struggling.
Be aware of the side effects, if any, with the gliptin.
I was on sitagliptin for a few years, but I decided to stop, as the effect of the gliptin wasn't necessary did to strict dietary changes.
But always refer to your GP, with the number of warnings he is getting on his libre.
Is the lows after food? As you say, his fasting BG is ok.
Best wishes