I hear your struggle! My son gagged for a long time too, (and had to drink water to wash down every bite) but no longer does it. My suggestion as far as food is to see if your local grocery store or health food store has a section with gluten-free items. There are alternatives to his favorite foods. Instead of cheerios, we use “Perky O’s” made by Perky’s 100% Natural brand. I think it is this brand that also makes a cereal called Nutty Rice (no nuts in it though). It is not like Cheerios, but so good that it is worth mentioning. For pasta, there are lots of brands that make rice pasta – we particularly like the rice pasta (in spaghetti, penne, spirals) made by Mrs. Leeper’s brand. I use coconut oil on the pasta with a bit of Celtic Sea Salt and it tastes great! There are also many gluten-free breads. Food for Life makes great gluten-free bread and has many choices. Also, Enjoy Life brand makes fantastic gluten-free cereals and snack bars. We use Rice Milk – make sure it is labeled as Gluten Free. We use Rice Dream, Original Plain. We do NOT use the flavored or enriched because these have items which have been shown to be excitotoxins and harm the brain. (See writings by Dr. Russell Blaylock and Amy Yasko).
As far as cheese goes, I haven’t found a casein-free cheese. Most soy cheese contains caseinate or casein. Also, most DAN doctors advocate not having soy because it is problematic for children with autism.
I would then start slowly incorporating different tastes – just a bite at at time of other foods. Try some yummy vegetables like butternut squash with cinnamon and a little bit of stevia for sweetness. Or, cauliflower steamed very soft and mashed with oil, a touch of rice milk, and salt. For a long time, my son did not like the texture of red meat or chicken so I created recipes for gluten-free, casein-free meatballs and chicken meatballs (or hamburgers). You can also make them egg-free with egg replacer. In the coming weeks, I will share some of my invented recipes that are easy (15 minutes or less!), so check back here.
In addition, and I believe most importantly, what I did for my son was to address the physical causes for the gagging and food aversions. I obviously do not know what is causing these problems for your son. I can only share with you what worked for my son.
I put my son on a high-quality multivitamin and mineral supplement to address deficiencies. I noticed that this helped with many of his sensory issues, including his aversions to foods (ex. he then liked avocadoes, which he previously choked on). I also found that one of the most-helpful things for my son early on in treatment was digestive enzymes, taken with everything he ate. They helped him to actually digest the food he was eating, and his behavior relating to food dramatically improved within three weeks!
It is a struggle, but keep trying. The extra effort is worth it. Now my son gladly eats everything I give him – he even LOVES swiss chard and broccoli!
Blessings and endurance to you as you press on!

“Therefore, my beloved bretheren, be steadfast, immovable, always abounding in the work of the Lord, knowing that your toil is not in vain in the Lord.” 1 Corin. 15:58 NASB