Hello! My son who is also autistic had lots of feeding problems, particularly at a young age. At one point he would only eat orange food and no white etc. We found that the common thread of things he would eat were things that have strong flavor. We gradually expanded his foods by introducing one thing at a time that had a similar quality flavor-wise. For example. He loves vinegar-based things. We started (if you can believe this) giving him caviar and crackers in extremely small amounts just to see if he would eat that. And, that was only on rare occassions when we'd have a grown-up get together and happened to have caviar on hand. He also loves pickles and sourkraut. We knew we'd achieved something when he ate the kraut because it's white!
My reccomendation is to see if there's a common thread among the things he likes and expand from there until you're further and further away from the flavor-base. You might be surprised!