look into an approach called SOS (sequential - oral - sensory). It is a gradual approach that starts with the foods that your child will eat and gradually introduces new foods by choosing ones with similar sensory properties (texture, color, shape, flavor, smell, etc). The key with introducing new foods is to go slowly and be persistent.
Be prepared to present the same food over and over, with no pressure to eat it. The only requirement would be that he can not remove it from his plate. He may first grow to tolerate the sight of it, then the smell of it, then become brave enough to feel it, then lick it, then bite it, then eat it. This could take weeks, but if you remain calm, like it's no big deal that the food is on his plate and let him progress at his own pace, he may warm up to new foods.
The most important thing to remember is not to assume that turning his head, or pushing the food away means that he will never eat that thing. Just back off, and think 'slow and steady'.
To this day, I still separate the "filling" from oreos and similar cookies. I collect it in a wad and eat the stuff separatly. Things like chocolate chips, nuts, filled chocolate cups, etc in ice cream, are usually picked out and eaten by itself free from the rest of the ice cream.
Things with fine gritty texture, like grits..., and drinks with firm things like fruit smoothies, pulp in lemonade/orange juice, yechhho... it gets spit out the moment it enters... Surprisingly I can eat coscous and drink starbucks non caffinated frappichinos (however that's spelled) but I think it's the case flavor wins over disliking texture. If it's got adequate spices, I may eat it. Also I don't "drink" the starbucks frappichnios. I chew the stuff as if I'm eating ice cream. For me drinking stuff with floating bits...ick. Pudding and jello, please refrain from putting stuff inside... I'm not really a fan of shredded coconut put in stuff either.
My son has autism too Ann. We could not get him to eat anything for years and years. So I fed him what he would eat, like crunchy stuff. Peanut butter on crackers, french fries, chicken nuggets. He would not touch anything creamy. Don't worry too much about it. My son is 13 now and he surprises us all the time with the things that he tries. I believe that just like every other sense, kids with autism have a heightened sense of taste and the textures are just too much for them. Try simple crunchy foods. Good luck!
Sometimes it's the texture. Sometimes it's the "mixed texture" nature of some foods (think fudge with nuts). Sometimes it's the complexity of tastes (some kids disassemble a sandwich and eat the individual components seperately) . Sometimes it's the presence of some individual ingredient that they can't stand (like garlic or cinnamon).
Ann, your boy probaly knows these 3 things are "safe" for him and he can't go wrong with sticking to these 3 thingfs.
Jay, I think it may be the texture.
what are the 3 things he eats? my son isnt that bad of a picky eater but i know there are certain thinks like yogart, pudding, oatmeal or anything that looks like he wont even look at.