Have you tried different textures like beef jerky? When Erik was young and not getting enough fat/protein, i started making him sausages every day and he finally started gaining weight! Mixing in small pieces of chicken/veges with his rice or pasta. Not letting him have dessert or ice-cream until he eats his meat etc. I grew up with 11 brothers and sisters and we had to eat what ever was made for dinner, all of it. My mom made us try everything at least in small amounts. I did the same with my 4 kids and they all eat a variety of foods, especially my child with autism who was the toughest with food issues when he was young.

Quinoa in smoothies! 6 grams of protein in 1/3 of a cup! My picky vegetarian 13 even drinks it!

A friend of mine has skinny kids (non ASD), and she sometimes gives them protein powder from GNC store in their chocolate milk. You might need to ask the doctor because there's many different powders in the store and it's formulated for adult weight gain.

My son would only eat sauces at 4yrs old and I tried everything. I would give him vitamins but felt bad but on his own he improved when he was ready now at 5 he eats veggies meat everything. I think they have to be ready no matter what you do. Just give him vitamins liquid if he will not chew them as my son would not. I felt bad but at least he got some type of vitamins in him until he gets to the point to try new foods.

My 19 year old son has always been a picky eater. But over time he has improved tremendously. Have you tried Greek yogurt with fruit in it? Does he like any kind of fruit? When my son was young I could get him to each peach yogurt. When he was in elementary school I noticed that if another child suggested he try a new food he would be more likely to try it than if I suggested it. He's always preferred hot food rather than cold. He's always loved spaghetti and meatballs. So when he was in elementary school there was a while there when we packed him spaghetti and meatballs in a hot thermos for lunch. I remember the first time he bought lunch at school. It was one of those round school pizzas. His aide had taken him into the school kitchen in the morning and shown him how the pizza was made. When he actually ordered the pizza the whole lunch line of kids cheered for him and the lunch lady behind the counter treated him to an ice cream to show him how proud she was of him for trying something new. So keep encouraging and supporting. Try having him help you to cook something and/or show him how it's made.