My 27-month-old does not chew. He is still on the 3rd stage baby foods or equivalent. We now go to occupational, feeding and speech therapy. We've just started in the last few months because it took us a long time to get the referral for the feeding evaluation because we kept his weight up, which was stressful for all of us. In fact, when we met with the nutritionist there and she saw our initial sheet of a week of his diet she was surprised to find that he was getting most of the nutrition he should be getting, including enough protein. We really worked on it. I now think that "chews food" should be a developmental milestone for which they check, and if a child can not chew food by a certain age they should be referred to therapy immediately. I wonder if we'd started earlier if we might have better progress by now. I've seen one-year-olds pick up their birthday cake and chew it; no one had to teach them how!
I've been told that there is an intensive Pediatric Feeding Disorders Program in Atlanta at the Marcus Institute (named for Bernie Marcus, founder of Home Depot): http://www.marcus.org/clinical/pediatric_feeding.html We are not at that point, but it was mentioned to us by the relative of someone who had gone there and had great results. The child was eating maybe 5 foods and now eats anything. For what that's worth.
Have you ever looked at the book Just Take a Bite? (Meaning don't say "just take a bite" actually for children who are resistant eaters.) Maybe there would be some tips in there that you did not get in therapy. Our therapist was interested in it when I told her about it.
Our latest thing to try it to use a dropper of some sort to put food (such as yogurt or pudding) in the side of his mouth to force him to use his tongue to move it around. Using a spoon to feed pureed foods generally puts them in the center of his mouth so he does not otherwise get that exercise. Of course, that's just an exercise with food, not the way to feed him his entire meals, as I don't think he'd eat very much that way!
What have you tried so far?