T.M.
Have you tried a gluten-free/casein-free diet? My son also has sensory integration disorder and eliminating these from his diet has helped a lot. There's a fantastic place you can order gf/cf/organic meals and they deliver to your door! mcgovernsorganics.com.
Also, if your son likes pasta, we have found the Tinkyada brand (at Central Market or Whole Foods) to be really good. It's brown rice pasta, so there's no gluten, but you'd have to check for the yeast issue. Is he on probiotics?
Good luck! I know when we went gf/cf about a year and a half ago, it was hell. But it seems so doable now. You can do it!