My heart goes out to you, OP. This was DS2 (also autistic) for the first 8 years of his life.
Can you start by looking at what he will tolerate and seeing if you can disguise additional nutrients or calories in that. Ds would drink milk, so we added abidec vitamin drops to it, and splashes of cream - not enough for him to notice the change in taste but enough to increase calories.
He'd only ever eat two bites of anything then flat refuse, so my rule was the first two bites have to be calorie and nutrient dense. I did tiny postage stamp sized sandwiches of wholemeal bread with cream cheese instead of butter, peanut butter and jam. Cubes of chocolate cake made with almond flour not plain flour, butter, cream and extra eggs for protein.
I learned to offer a range of foods - never force anything, but explain what each type of food did: Protein makes you strong and is good for the brain (meats, cheese, fish, eggs) vitamins make you healthy and happy (fruit and veg) carbs give you energy to play, liquids keep your body and brain working properly. I'd ask him which from each frood group he liked, and then he could have whatever he chose, even if that was a chicken nugget, two mouthfuls of rice krispies and one carrot stick. But he'd sit up to table with us, even if all he'd eat was ice cream, so he got the social side of eating as a family.
He liked fast food and food packaged to appeal to children. Give in to this - at least it gets the calories into them. Sweets and crisps are better than nothing - ignore frowns from yummy mummies who have never dealt with ARFID. Once you know what he likes, you can start making your own healthy variants of it. He'd eat Mcdonald burgers so I started to make mini burgers and then sneaked pureed veg into them.
I used to sneak food into him while he was focused on Tv or a game. Basically everything that is a bad idea for fostering good eating habits in normal children, is a good idea with ARFID: fast food, junk food, eat while watching Tv etc. All better than starving to death.
Is any of this helpful? Might any of this or similar, work for him?
DS too dropped off the centiles. I will never forget the health visitor putting a biro mark in the margin below the lowest centile. I cried afterwards. It wqas one of the most stressful times of my life and it went on for years. But he did, eventually, grow out of it.