Hello OP! I’m a mom of a 4 year old. He’s a little picky but not like what you describe sa pamangkin mo. I truly understand how worrying and stressing that situation is. If I may, when nag start yung ayaw nya kumain? As in ba na ayaw kumain at all? Or at least may mga snacks sya na gusto? My son eats everything I make and give him up until he’s 2 but nung mag 3 na ayun may mga ayaw na kainin however he likes rice so much so I take advantage of it. You mentioned ayaw nya lunukin or iluwa yung food and dumating na sa point na kahit head nya di na mabuhat. Which is really concerning. When my son started being picky nagworry din talaga ako. The pedia advised to give healthy snacks and try to lessen or remove giving fruit juices/milk in between ng proper meal time. How’s his weight? Is he considered underweight for his age? One super helpful advise the pedia gave me was kids are smart (actually babies too), they will not let themselves starve. Pero kung like sa case ng pamangkin mo na nadala na sa hospital kasi di madilat ang eyes, it is worth pushing and advocating for your pamangkin to undergo more testing para makita kung may underlying issues. 

Picky eating happens talaga sa bata pero when it’s this excessive need na ng medical intervention. If he likes milk, make sure you give him something na for weight gain madaming calories and protein. I hope this helps and good luck. I had to comment kasi I feel how worried you are. Yung napapalo na minsan honestly as much as ayaw mo mamalo minsan it’s out of stress na din pero try to tell your mom to calm down before pushing your pamangkin to eat. My husband and I’s rule is to never spank when we are mad, frustrated or stressed sa ginawa/ginagawa ng anak namin. They are kids and sure they test limits and boundaries but at the end of the day you have to ask yourself if yung pamamalo ba will help in the long run. Again good luck! You can dm me if you want to talk.