I grew up with my brother and two sisters. We always were hungry and if you didn't want to eat, one of my siblings would take care of the problem. If there was something new on the table we hadn't seen before (happened quite a lot) we tried it anyway because if we didn't there wouldn't be any left.
I always thought kids would eat if they got hungry. Then, my youngest had a baby. My grandson was diagnosed with "failure to thrive" which is doctorese for "we have no idea." He wouldn't eat. He was never hungry. He was tube fed and seems to have gotten a little better but, remains a picky eater.
What the hell is the difference in our environment between now and when I grew up? It seems like every other kid today has some kind of allergy or food intolerance today. Lactose intolerance is very common. When I was little, there was exactly one kid in the entire school who was lactose intolerant. I always tried to sit next to him to get an extra milk.
Failure to Thrive is a symptom not a Diagnosis. There are many things that cause failure to thrive (basically acting in a way incompatible with life). There are plenty of causes I am sure we don't know.
Technically "Idiopathic" is doctorese for "i dunno". When a Doctor says you have "Idiopathic" anything it means you have that thing and I have no fucking good guess as to why. Sorry.
I am sorry about your grandson and truly hope he does well on the road to recovery.
Here is the thing.
50 or 60 years ago when you were an infant, if you had failure to thrive you most likely died. In fact you probably didn't even survive until birth. We forget that both infant and maternal mortality rates were absurdly high not even 100 years ago. There are a lot of very sick babies who are not only making it to term but also surviving and prospering that would have had no chance back in your day.
As far as Lactose Intolerance... No mammal is made to drink milk past infancy. Especially not the milk of a different species. There is a reason you have to give formula to infants if you don't breast feen and why you can't give cow milk to infants. It just doesn't have what you need. The only benefit cow's milk has is calcium (which breast milk is better at) especially in adulthood. We need more calcium in our diets and milk is pretty good and it can be fortified with Vit D for an extra boost so we keep drinking it. If you stopped drinking dairy products right now you too would become lactose intolerant because our bodies know we don't need it and as soon as we stop sending the stimulation we shut down the enzyme/protein production that is required for digestion.
Now for Allergies and things like Irritable Bowel Syndrome. That is a whole crazy monster in and of itself. True Anaphylatic allergies are just shitty luck. Seasonal Allergies and IBS and maybe even autoimmune are multifactorial of course but I am willing to bet that in the next few years we are are going to be making some huge connections with our emphasis on "over cleanliness".
Frankly, we aren't letting our kids get exposed to germs and bugs and worms. Our Immune system is like the T-100. It is a learning computer. It sees something it doesn't like and it kills it. If it sees it again; it will kill it again. People today are germaphobes. Everyone. Some much more than others but don't pretend like you don't wash your hands with antibacterial soap throughout the day and socially isolate and quarantine any friends who have the sniffles. Our bodies have spent the last few thousand years adapting to the fact that there is tons of shit that thinks they could do pretty freaking good in a nice 2 bedroom apt in your sinus cavity. It is constantly vigilant and ready to sweep them away like germs were the homeless and they are Guiliani Era NYC cops.
This is especially true in childhood. Our bodies are ready to fight at a moments notice. So like the Drunk meathead pounding Yager at the end of the bar, if no one shows up ready to rumble he is still beating the piss out of someone or something regardless of what damage is done. No surprise that not shitting your brains out every other week from spoiled Dark Ages meat leads to our immune cells in the gut mucosa getting bored and start beating the shit out of the gut lining.
Want to have your mind blown?
What is the Antibody in our body associated with Allergies and Allergic Reactions? IgE.
What Antibody in our body is the main defense against parasitic worm infections? IgE.
Mother fucker. Imagine that. When was the last time you heard somebody's kid had worms? How about allergies? Surprise Su-fucking-prise.