He needs to be feeding himself.
All the 'knack' stuff, pretending to eat, feeding him spoonfuls, is ridiculous and not sustainable or advisable.
Buy a big oilcloth and put it under the high chair. Put food on the high chair tray, about one teacup total volume at a time. He could have scrambled egg, cut up fruit, peas, diced carrots, little bites of toast, small pasta shapes, cubed sweet potato or white potato, cut up fish fingers, even little cut up bits of apple or pear without the skin.
Don't overwhelm him with a tray full of food.
At breakfast and lunchtime he can have a chance to use his own spoon and eat spoonfuls from a bowl while you are having your own food at the same time.
You have to stop spooning food into him yourself, and try to get a handle on the anxiety you feel about him getting enough and how that's related to his sleep (your opinion that they're related may or may not be true).
You need to relieve your H of feeding duty. It's not working. It's causing them both stress and setting up a bad pattern.
He can pick up and tidy the home or get some cooking done while DS is having his evening meal.
Or all eat together earlier - DS will copy you as you feed yourselves.
Or DH can do bath time and read a story or settle him for bed.
Your DH needs to stop being so touchy about all of this and read some books on child development.
Insisting on doing things his own way for several months when it's clearly not working is not a positive character trait in a parent.