My son (aged 5 and with ASC) is seen at St Thomas hospital for his eating. He literally ate nothing until he was 2 - surviving on milk. He still only eats about 10 things. I know it's really hard not to worry but it's amazing how little food our kids can exist on.
At the hospital they told me about a girl they're treating who only eats two strawberry yogurts a day. And she's fine!
A few things they've told me over the years that might help you - apologies if you know all this already.
Don't make meal times a battle/ withdraw the few things they do eat. In a battle of wills, our kids are always going to win! They treated one child who only ate porridge. The parents thought by not giving the porridge it would force the kid to eat something else. After 5 days of the kid eating nothing, they gave up.
Put a small amount of whatever you're eating on a plate in front of them everyday but don't make it an issue if they're not interested. If your son won't sit at the table (mine won't), just leave it out for him.
I have to feed my son in front of the TV to get him to eat - think the distraction helps. If it was up to him, I don't think he'd eat at all as he never says he's hungry.
I bought a divided plate for my DS as he has an issue about foods touching. That has helped a bit.
The hospital gave me a social story to read with my DS. Google 'social story eating'. We started really slowly by getting him to smell food, then touch it before trying to get him to eat. My son is hyper sensitive to smells and textures and these issues are really common for people on the spectrum.
My son's eating has got better as he's gotten older. At the clinic they say that some kids improve around 5. They don't really know why.
If you get a diagnosis of Autism, you may get some specific support around eating. As I said, I know how hard it is not to worry but hopefully things will improve.