What about the behaviour, how do you prepare the food, the settings, your approach...
Have you tried having him/them with you, from beginning to end, from browsing a cooking book together - I know at 1 he doesn't know what that it is, just glossy pages - but look at pictures, even if not the one you will cook. Go together to the shops, and pick food together, than go home, have him with eye levels on kitchen tops, and give him a wooden spoon and a plastic bowl, and other safe bits and pieces, while you chop a carrot, add onion to a sizzling pan, and prepare whatever you are cooking. Have him watch you tasting something, adding fresh herbs to it, give him a big chunk of vegetable to play with while you finish and move together towards oven and let him see you from far away put something in it.
Do not offer anything .
Then when it is ready, close by again, and adjust herbs, spices, and take a fork for you , a baby fork for him, and let him see you tasting, and then hand him his fork while you turn your back at him.
Eating is multi senses , you see, hear, smell, touch as much as taste.
Human behaviour is also a lot of copying, so his big brother can be a model, but without pointing it at any of them.
I know you are super stressed and just happy he eats something, but real food can't compete with baby junk food, the melty sticks (it is corn flour and oil , just like Doritos or Cheetos minus the salt, with some dried veggie powder so they can put a veggie on packet) , crips, and ritz cracker . Could you not give them for two three days? They are your antagonists here.
You need to be able to offer something that tastes palatable and comes close to those artificial combinations. Have you tried a buttery risotto with real parmesan cheese? It will offer the same ratio of carb+fat he is now used to with the same soft texture that melts in mouth. Or some roasted pumpkin in the oven with olive oil, and a bit of salt now that he is older. If you leave the pumpkin long enough, it will caramelised.
Careful with drinks or snacks just before meals. Juice, squash, ... will kill appetite. Only water 2 hours prior a solid meal.
Do you all eat together at the table, the four of you, no tv, no screens? WIth the food in a main dish at the centre and you serve it from there into plates, your DS1, DH, yourself and then wait for DS 2 to react to food being served and you make the gesture towards him, so that the requests comes from him
Have you tried using preloaded baby forks?
Hold off the melty puff and crips if you can and similar ultra processed food such as baby rusks, baby rice cakes , baby wafers,... just for a couple of days and increase milk if you need to. Ultra-processed food are totally different sensorial food, they have nothing with hunger but pleasure centre, they mess up everything.
Can you make your own focaccia at home? or other herbed flatbread? You will need some saltiness, because powdered veggies, having lost all water, have higher mineral content per weight, and those minerals include sodium, magnesium, .... that give a saltiness without added salt. Use dry thyme, dry oregano, dry rosemary or premixed herbs but watch out, some brand add sugar. Dry fruit on the other hand is a way of adding sugar without adding sugar, so the food becomes a biscuit.
It is very complicated to unwean from artificial food toward fresh, but you still have a window of opportunity. You have to find the food that clicks for him, my guess would be something close to the macros ratio from the baby junk, so carbs+fat+salt/sugar. Have you tried to make crêpes, which don't have sugar in recipe (250 flour, 3 eggs, 1 tbls oil, pinch of salt, 500 ml milk) , and then you can on a tiny piece, put a very thin layer of jam or your own apple sauce.
It is so hard when they don't eat. I hope at least one of my suggestions will help.