That sounds really hard. I have had phases of food refusal albeit not as extreme. I’ve tried to keep mealtimes as relaxed as I can, not really look at her whilst she’s eating and not stress if she doesn’t eat much. Just chat to her about the day etc. I also try not to give her any snacks in between meals and plenty of exercise so that she’s got a decent appetite. She definitely eats more when she’s ran around outside lots.
Does he show any interest in pastry? My girl likes to eat the pastry off the tops of various pies I make, then gradually has worked out that the inside is alright too so now eats quite a lot of her food in pastry topped form! Beef stew pies, chicken in cheese sauce pies, lamb stew pies. sometimes she eats basically gravy soaked pastry, other days will eat more of the filling.
What about yoghurt? Can stir in some nutritious additions eg ground almonds or chia seeds. Stewed fruit. Same with porridge, if he’ll eat that you can stir similar things in, or even scramble an egg into it whilst it’s cooking for extra protein.
a little blob of ketchup squirted on things (can get a reduced sugar / salt one) can make things more appealing. My girl ate a veg and cheese omelette yesterday that had a bit of ketchup on each piece, and she’d consistently refused veg in her omlette previously.
Food served in a big dish in middle of table sometimes works well for us too. E.G. a roast dinner - she will sometimes not touch the chicken on her plate but she’ll see dad tearing a leg off the bird on the table and she’ll reach out for it and will eat the chicken from centre of the table.
She won’t touch most veg on her plate at mealtimes but if we play “tea party” and I bring out a little plate of raw carrot or something for the bears then she gets quite interested and will try it and often eat quite a bit of it. Maybe she feels less pressure in that situation.