My cousin refused to eat solids.
She got to the point she could pour her own bottle of milk and put it in the microwave, aged about 5yo.
They tried adding flavours to the milk, treat food, ordinary food, food off their plate etc.
Not a crumb passed her lips.
Eventually what they'd do is for a meal they'd sit her down with a small amount of what they were having and let her play with it. She rarely even took it to her mouth.
She was perfectly healthy, just wasn't eating food. I think the doctors gave them some vitamins to put into the milk, but other than that, she had nothing except milk.
Then one day, probably about 3yo, we'd taken her to feed the ducks and given her the bread bag to hold. After feeding the ducks for a bit, we noticed that she had a piece of bread and was eyeing it up. Eventually she took this tiny little bite - and ate it.
Nothing was said, although I remember my uncle laughing about all the times they had tried chocolate spread/jam/anything and there she was eating a bite of stale bread.
That was the beginning of a very long process. After that she'd occasionally put a tiny bite in her mouth at lunch time. They wouldn't comment, just tidy up what wasn't eaten at the end. Then she was at a friend's house and they found something she'd eat. Smash.
For a few months she ate smash consistently. Not real mash, just smash.
Then she started having little bits of ham.
When she started school she'd eat ham, smash, chips and I think it was frozen peas. But very quickly she started eating more and soon she ate like any other 5yo.
She's now an adult, and I can say that she does not have ASD, or ADHD and eats a good varied diet. In fact, I can't think of anything she doesn't eat.