Mil is the reason my son won’t eat!!!

So bit of a backstory I had a baby girl in June who was unfortunately born very poorly and required an admission straight to PICU in a total different city after birth and then 2 open heart surgeries within the 4 months that she had been in hospital. During this time, my MIL has been on and off minding lay my son who is 15 months. She’s essentially fed him all the frozen stuff that she’s brought her children up on like nuggets, ready meals, fish fingers, spaghetti hoops etc. sweets and chocolates on a regular basis despite their being plenty of healthy meals that my son liked. We bought our baby girl home last week and now he will not eat anything that we cook, like vegetables, curries, pies etc all things that he used to eat and enjoy. He flat out refuses and will scream bloody murder. I had him weighed yesterday and he’s stayed the same weight as he was 4 months ago too :( whilst I’m grateful that she has minded my son during the times I’ve not been able to, I’m also very frustrated that I now have to battle a toddler every night who won’t eat anything I offer him unless it’s nuggets or freezer food 🙃 since myself and my partner do not eat freezer food, it annoys me more. She doesn’t see an issue in the diet she has supplied and seems to think it’s perfectly acceptable. Before anyone says “at least he was fed” I get it but it’s kind of not the point… he was fed absolute crap that no 15 month old should really be eating UNLESS they have sensory issues/ASD and it’s their safe foods. How hard is it to literally reheat a decent meal that I’ve prepared in advance? Anyone know how I can get him to eat the things he used to once enjoy? I really do not want him to be a chicken nugget child 😬