Thank you for your email. I apologise for the delay in getting back to you. I understand how stressful it can be when your little one doesn't want to eat.
From what you have described your son is receiving sufficient nutrients. He could live on his formula alone and be healthy, so you don't need to worry about whether he is receiving sufficient nutrients at this stage. He is however, essentially on a liquid diet receiving over a litre of liquids a day which is where you can make changes.
If there is no specific reason for him to be on NaN2, you should begin weaning him off it as it is very filling. Start by mixing half Nan2 with cows milk. There are various milks available in stores which are specifically designed for children his age. Also offer him smaller amounts of milk/formula. He is probably having his milk/formula when he sleeps, so giving him 150ml,100ml, 50ml, should still be sufficient to help him sleep, but not enough to fill him up. If he is unable to have cow's milk, then give him soya milk, or water down his formula, but eventually get him off it. Also dilute his fruit juices.
Secondly decide to be calm and patient when it comes to meal times. If you are anxious, stressed and towering over him, he will also become stressed and unable to eat. By not making an issue out of eating, food will not become an issue for him.
Always offer him food at breakfast, lunch and dinner, even if he just plays with it, and let him feed himself and mess somewhere where it doesn't matter like outside or in the kitchen. Have healthy snacks available around the house and in your bag for him to nibble on such as carrot sticks, dried mango, apple slices, watermelon, packs of cream cheese and bread sticks to dip in the cheese. Always keep an eye on him so he doesn't choke. Make family meal times where you all sit down and eat together fun. He will copy you and dad, so if he can see meal times are enjoyable, and he can try things off your plate, he will be encouraged to eat solid foods. If he ends up liking only a few foods that is fine. Children have over 30 000 taste buds compared to our 3000, and therefore are very sensitive to foods with a strong taste. This was to protect them from poisonous foods during prehistoric times. Substitute the bread and butter with fish fingers, chicken nuggets, spaghetti bolognaise, ( messy but great fun to shlurp. teach him how to shlurp the spaghetti and you will have good laughs.) Chicken drumsticks- let him chew on the bone after you've cut up the chicken. (Remove the thin bone on the drumstick). Fruit slices, carrot sticks and peas.
Don't give up children can change their eating habits from one day to the next. Generally they will seek out what their body needs and studies show that given choices they eat a balanced meal by the end of a week. Keep offering him foods , but never force him to eat. If you are concerned about his health or if he shows a lack of energy or developmental delays in any area then chat to your paediatrician.
Good luck and let me know how it is going.