You have to find a speech language pathologist who is an expert in feeding therapy. With this level of adversion only a trained professional will be able to help you. Thankfully he will take the protein powder. Also call the office again and see if they can get you in sooner. You don't want to be obnoxious but the squeaky wheel gets the grease

Try adding the powder to other things with the liquid texture and then maybe add it to milk which is a bit thicker. He might be more willing to try things if they taste like the powder he is used to. Keep giving him different textures, even if he spits them out. My son would spit things out and gag on everything except milk.