chicken nuggets, sausages, crisps and yogurt, cereal and maybe pasta sometimes, Juice
There is plenty of protein in there, dairy and carbohydrate, vitamins in the juice, and in the milk on the cereal.
Nothing to worry about.
Just go easy on the crisps due to salt content or get some low salt children's variety. Buy the best quality nuggets and sausages you can afford.
Above all, take ALL the emotion out of mealtimes. No fussing, persuading, cajoling, making games of eating, none of that.
Imagine he is an adult guest. You all sit at a table together (assuming a DP/ family but can be just you and him e.g. at lunch). You present a plate with food you know he likes (plus a tiny bit of whatever you are having), then you talk with him and each other about anything else except the food. Then when the meal is naturally over, clear away the plates with no fuss and no comments.
Don't keep looking at him during the meal, just a glance now and then but try to keep it natural and normal, no special attention. If it is only two of you, glance at his face while you chat but not at his plate.
However little he has eaten, don't offer 'extras' because you are worried that he hasn't had enough. Instead, perhaps make a family tradition of having a 'supper' of snacks or crackers two or three hours after the evening meal and before bed (can be on the sofa or more informal), and again make sure you all eat together, so he is not getting anything special just for him.