Try a little of different foods and see if there are any allergic reactions he may feel uncomfortable. Many kids in the autistic spectrum have gut issues... It would be helpful to look into any physical/ biochemical reactions he has with certain foods and to avoid them. He will eat what is comfortable to him, but you will have to find out what they are, and I know it is worrisome and hard {{{hug}}}
My child is also on the spectrum. She was allergic to many foods and had a lot of rashes. She was diagnosed with inflammatory gut and used to have diarrhea also. But after changing to gluten and casein free diet, and avoid moldy foods, the condition healed and improved through time.
Her diet was limited in the beginning, so as she grew up it was difficult to feed her; but I can say she is now very healthy and eats very well. There are foods she is not accustomed to and doesn't eat them much, e.g. sweets/ desserts :), diary (except she likes cheese once in a while), eggs, wheat (although she can occasionally eat pizza, ramen, bread without any ill effects).
Her usual diet consists of rice, different kinds of meat, rice bread with cashew nut butter, rice noodle, veggies, snacks of 'good' potato/corn chips, taco..., Martinelli apple juice (don't use moldy apples), she can now eat even the limited foods with moderation and rarely has reactions.
P.S. 1) We used to buy pizza rice dough from Whole Foods and bring it to any pizza places and asked them to use it for cheese pizza for her.
2) Ramen, the good kind from the restaurants, not the instant noodles full of MSGs and chemicals.