Finding nutritious safe foods is vital. People super into weights often eat a pre portioned amount of rice and chicken and all the spinach they want, and my autistic wife enjoyed doing that when she did it. Texture is huge but so is familiarity. For western autistic people safe foods tend to be heavily processed "kid foods", make sure your kid's idea of comfort foods isn't that.

Also encourage him to be brave and try new things with the safety of it not being commitment to the food, and as he's older talk to him about what he likes and doesn't about things and check in before trying something new when he's old enough to. My wife has days where she has the mental energy to try something new, and days where she doesn't. Additionally if I can describe what it's like it takes less energy for her to try. And don't pull the "it's just like chicken" thing, go more "these fried mushrooms are tender and slimy with a strong umami taste, kind of like soy sauce but not as salty or fermented tasting"

Don't take no for an answer. What's for dinner is what you get or you can go to bed hungry. My mom just didn't put up with my bullshit and let me not eat my food if I didn't want it but absolutely refused to let me have anything else until dinner was eaten. If I ate my dinner I could have whatever I wanted (in the house, which was pretty much all healthy food).

I have a daughter on the spectrum. It took many hours each week and many weeks in a year of ABA (RBT, hopefully getting my acronyms right) therapy to get her to accept a variety of foods. Fast forward to a few years later, she'll eat anything including spicy food (by her own choice). It was definitely a texture thing, as well as her wanting a constant she could reliably predict. As part of her therapy we always included something she liked alongside the things she didn't. We were firm she had to try everything every time, even if she couldn't finish.

You got this!