if you really want to get him to eat have a bucket at the table. Throw away all the junk food and only present typical family food appropriate to the meal. (Avoid spicy/flavorful and mix textured food. Kill the taste of green vegetable with butter or sliced almonds) Make him eat some and let him go hungry if he won't eat a typical amount. He will not starve. Most OTs do they play with the food nonsense and several years later have pathetic results. The minority that engage in the "old fashion" method have the kids eating a normal (but somewhat bland) diet in 2-3 months.
It may very well just be sensory related...my 3yr old DS who is NT has a severe sensory processing disorder. Many textures are hard for him to deal with & can cause melt downs if he's 'forced' to touch/eat it...he even gags/vomits with some textures like bread/pasta. He also has problems with hair cutting/brushing, fingernail/toenail cutting, finger painting, play doh, sand, etc...his diet consists of dry cereal (mixed foods are typically an issue), pop tarts, muffins (this is the rare bread item he can handle), pb crackers, apples, grapes, banana (if the peel is left on so he doesn't have to touch it), chicken nuggests, crispy plain hamburger, crispy bacon, crispy fish sticks, french fries, & chips/pretzels/popcorn. As you can see, most of his foods are ''crispy/crunchy'' textures. Try overcooking some foods to give them that texture & see if that helps. I wish I could say it gets better with therapy/etc but honestly I've not noticed much diff. in my son...other than he can handle seeing & sometimes touching it now. He used to cry if there was a ''no no'' food on his plate...he couldn't even handle the sight of it. So I suppose therapy is helpful...I'm stiill hoping to add more variety & the best thing you can do is what his therapist told us: never force anything b/c that will cause more anxiety; offer the same new food everyday on his plate w/food he likes & remind him you aren't going to make him eat it (this desensitizes him to the food); start off by having him touch it w/only a fingertip & slowly move up over days/etc to tasting; if he handles a new food on his plate w/little melt down, praise him; if he touches a new food, praise him; if he tastes a new food, praise him.....hope that helps!