tips for fattening up a skinny child

[Branleuse]

My ds2 was a big baby - 10lb and gained weight steadily as a baby. He was off the centile charts for height and weight. Lovely chubby thing
As a toddler his weight steadied but he was still tall. Now hes 5, nearly 6 and skinny as a rake. He weighs less than his younger sister and has visible ribs, kneebones. Hes positively scrawny.
Hes very picky about food and has a real sweet tooth, but i dont like giving sweets much. His dads a picky eater too, and ds2 has very similar eating styles and is just not that bothered about eating. Just grazes. Will not eat at all rather than eat something thats not his absolute favourite.
I cant reason with him very well as he is autistic. Ive always taken quite a relaxed approach as i dont want to make it an issue, but I cant say that im not a little concerned, even though hes happy and seems active.
Hes nearly 6 and weighs just over 17 kg (2stone 8). Average height.
he feels the cold terribly as well.
Ive just ordered milk poweder to add to glasses of milk to see if that helps, and i make his toast extra buttery, but he never usually finishes anything anyway.
He didnt eat ANY breakfast this morning. Had half a glass of strawberry milk and that was all he would have.

My frankly scrawny ASD son is 9 now and we are making a bit of eating progress.

We go for more frequent small meals/ snacks and its easier for him to eat at the table without everyone else. Breakfast is currently 1.5 wheetabix (tesco value equivalent) lots of milk, warmed for 30 seconds in microwave mushed and then he adds cake sprinkles or currently we have a tub of grated Christmas chocolates. He is limited to two spoons or he'd just keep adding without
thinking. DH and i are having a reduced carb diet so he usually has a finely chopped chipolata sausage as well. This is by far his biggest meal of the day.

For snack he takes a pot of home mixed dried fruits (sweet and very high calories for a small volume of food)

He has a hot/ warm dinner that i send him to school with. often a home made pasta (he likes making the dough and shapes with me) with a rich sauce. Its very easy eating. He has a fruit pouch. Jelly as well - this doesn't often get eaten. I batch make a months worth and defrost/ reheat a portion in the microwave so its not too much hastle.

Getting home is typically baked goods cakes/ flapjacks/ fruit bars he loves carrot cake.

Usual range of small evening meals - everything cut up small easy to eat. He eats blended veg soups well with tiny fresh bread rolls and grated cheese to sprinkle in.

Often toast, cereal or something extra if we have time before bed.

I think its a reasonably balanced diet but all portions are far smaller than i'd like himto have. Reducing the portion size right down has taken the pressure off him and he sort of accepts he needs to eat this reduced amount.