Could a child occupational therapist help? They might be able to figure out if it's a sensory issue or a muscle coordination issue. They also might be able to figure out how to help him after figuring out why.
My daughter doesn't like tough textures. She loves soft things like all the pastas (mac&cheese, tortalini cut up in small pieces, ravioli, spaghetti cut up in tiny pieces so she doesn't have to chew, lasagna cut up tiny, etc.). Oh, she also loves applesauce. Hotdogs, but cut up into tiny pieces (I cut them in slivers and then chop it into 1/4 after that). When I don't cut things up really tiny, she ends up craming a big piece in her mouth, not chewing, and then choking on the stuff. She's getting better. She is starting to chew more, but it does have to be soft food like pasta. I am not cutting the pasta up as small anymore. Oh, I did find out one hard food she will eat. She acutally adores chips. I think it must be the salt. And she'll chew those. They break pretty easily. :)
Is your child talking at all?
Have you raised your concerns through your paediatrician? There should be someone you can be referred to who can help you with these food issues. They should have experience about autism. In our City the Special Educational Schools and Nurserys have programmes for child with eating difficulties.
Do you think your child has any motor co-ordination problems? The reason I ask is that some children cannot co-ordinate their muscles properly and this can involve the muscles you need to chew and eat and speak. I don't know if it falls under verbal dyspraxia or whether it has another name. But it might be worth looking into as well, especially if there are other co-ordination or balance problems.
I presume that your child is okay with soft, pureed kinds of food?
Have you tried graduating from very soft to harder types of food eg. from a banana to a piece of cheese?
Will your child eat chocolate? Rather than chocolate buttons, try a cube of chocolate that starts off hard, but then melts. But I would also definately go to your paediatrician and tell him your concerns and get referred as well.