I know the food battle well my son will only eat the following
nutella sandwiches
pepperoni pizza
dry fruit loops
fries
orange juice
milk
and cucumber skins(yes just the skins) so my family doc prescribed pediasure he drinks two a day as some days he just is not very hungry maybye try is and see if it works for your kid but make sure you always offer real food first

My 10 year old is 65 pounds. We make him smoothies with protein powder (vanilla hemp from Trader Joe's), Benefiber, lots of fruit, a few baby carrots, Trader Joe's Vanana yogurt, and vanilla coconut milk. That is helping a bit. Sometimes I put the Trader Joe's french vanilla ice cream in it too. we feed him every fattening thing he will eat at night after his brothers go to sleep.

Both my kids are in the nebulizer when they get URI's. Especially my little girl. She is very slender, but unlike her brother, she is all muscle. I doubt the nebulizer has anything to do with it. My husband is slender..has been all his life(LOL-must be nice) and has tried all the weight gain shakes and working out to bulk up for years to no avail. Just like someone else said...it is all in the genetics. I was skinny for a long time...till I started taking steroids every few months for severe asthma.

My son is 8 and very tall for his age, but is so SKINNY his bones stick out. He eats like a teenager and is not super picky. He eats about six meals a day (breakfast at home, breakfast and lunch at school afternoon snack (to him this is ham and cheese sandwich or a can of pasta) dinner and then before bed, he like to eat a pb and j w/a glass of milk, or if there is any left over dinner (baked beans or green bean casseorole are favorites of his) or even frozen wheat waffles plain.
Does your son seem healthy otherwise? My son is very solid despite his appearance. At four foot 10 and half inches, my son only weighs 62 lbs. I just try not to make a big deal about it. Being told "you're skinny" all the time is just bad as being told "you're fat."

I have the same issues with my son now who is 7 not on te spectrum but has other issues anyway he is only like 38 pounds right now but have had him evaluated several time and just very skinny for his age, and yes he is like me a picky eater and even myself i am only 4'11 and weighs 100 pounds, so i guess the apple just dosnt fall far from the tree. my other children were like that too untill they reached the age of around 10 and then they discovered the joy of food and now they are at a healthy weight, I never tried to make an issue out of food in fear of developing eating disorders when they got older and come to find out they seems to settle the issues on their own.