Your DH was rightly worried that a bigger waist increased his risk of several nasty diseases.
So, he is now much slimmer with a flat tum. Usually good news.
Has he really lost too much or too quickly, or are you just used to / happier seeing an overweight DH ?
Is he under BMI 20 or under 18.5 ? That's important to check.
Maybe he now needs to go to the maintenance phase or even regain a little. Or maybe he is doing just fine.
Anyway, you refused to enable this change, so he chose to buy ready meals.
It's obviously important to him. If someone chose to go vegan, that might also disrupt family meal plans, but it would be wrong to oppose it.
The short temp and aggravation on Fast Days is natural if he feels you are trying to force him to eat food he doesn't want.
Equally, if he is nagging anyone to join him, that is also not ok.
Sounds like he is more anxious about health consequences of being overweight - and maybe even worried if you or the DCs are too ? - and you are worried about DCs seeing calorie counting. So you would also be worried about a standard diet of counting every day.
Most 5:2 mums just cook the same thing for all the family, then have a moderate (calorie counted) portion of the protein and veg, without the starches or pud. Their families don't seem to row over this.
Could this system work for you ?