I feel for you. I understand. My 4 year old is autistic, only talks in song from tv shows and movies, and has SPD as well. Our 5 year old has anxiety to anything new including food, and SPDs as well.
So, I know what you are going through. I am assuming that your son ins't allergic or intolerant to dairy because he eats Mac and Cheese. That makes your life SO much easier, you have not idea! Both of mine are intolerant to dairy and soy.
My guy eats anything dry and crunchy with heavy flavor. Garlic Triscuits, Movie Theater Orville Reddenbocker's microwave popcorn, home made popcorn with real butter once in a while. NO other flavors. Peanut butter ritz crackers, chicken nuggets sometimes, french fries, no other shapes, no jo jos, no chicken strips, a banana once in a while, apples, chips, Cesar flavored dried processed peas, Oscar Meyer turkey hot dogs (nitrite free) and cookies. His developmental pediatrician suggested Nutter Butters as a good protein source.
To supplement his nutrition we do a chew vitamin, probiotic gummy, and Viactivs. He also drinks Neocate, Jr as a nutritional formula. He uses this instead of like Pediasure because of the dairy and soy intolerances.
I would recommend seeing the nutritionist that works with Autism and your developmental ped. They understand the reality of what they will eat. She should be able to help you get a prescription for like Pediasure so that you medical insurance will pay for it. Or, WIC should cover it, if you are receiving it. (this is where we get the Neocate at $60 a can.)
No, he doesn't need meat, but he does need full balanced nutrition. If I can help, please feel free to PM me.
And, yes, Gamma is correct, a feeding tube is a possible reality! The formula our son uses is what goes in the feeding tube, he just drinks it instead. I worked with SPED students all last year, and 2 students with Autism did have feeding tubes as teens.