21 month old with PDD and eating?

My son was diagnosed with PDD about 2 months ago, however I have had a feeling he was autism spectrum since he was about 8 months old. Eating is his greatest challenge. He doesn't feed himself anything but Kraft American cheese and Gerber puffs but at one time he did allow me to feed him chopped up table food on a spoon. Noodles, french toast, shredded meat, mashed up meat, etc. When he turned about 14 months he started throwing up everything and anything and I had to revert back to stage 3 baby food. He then started throwing that up and I started on stage 2 baby food. Right now his diet is: Kraft cheese and puffs (as previously mentioned), Gerber baby oatmeal or rice cereal mixed with milk, applesauce, Handisnack banana pudding, stage 2 meats, Yo baby yogurt, Betty crocker mashed potatoes, and stage 3 Gerber dinners. Still, every once in awhile, he gags and throws up on his staples. I never know what to expect. He does receive therapy through early intervention 3 times per week, 2 times are for feeding OT. He is not responding to feeding OT at all and is making zero progress. He's dipped in the growth charts so his doctor prescribed a high calorie formula that is basically keeping him from wasting away. I don't want him to be dependent on this forever, but I know this is issue is common with autistics. What else can I try? Forget pureeing table food, he throws up on any texture that isn't Gerber's.