Dear L.--I know I'm really late with my response, but I just got back from out-of-town, and when I saw your request, I HAD to respond.
My son is also autistic and speech-handicapped. He's 27 now, however, and we have no eating problems with him at all--unless you call his wanting to eat all the time a problem.
Anyway, when he was your son's age, he, too, was very rigid in what he would eat. He wanted the exact same for lunch and dinner, no matter what the rest of us were eating. His pediatrician told me not to worry, but just let him have whatever he would eat, even if it was ice cream three times a day.
My advice to you would be: Let you son eat whatever he wants. As you have already found out, he will tire of it. Now, how to get him to try new foods: Tell him that someone he loves/admires wants this food and said he could have some. With my son, it was his great-grandmother. I would say, "This is Mama Tora's ham. She said only you could have some." He bought it hook, line & sinker. It didn't matter if the rest of us were eating it, too. He was super-impressed that she wanted him to have it.
To this day, he talks about "Mama Tora's soup" and "Mama Tora's spaghetti"--you name it.
I hope this will work with your little one. The only other thing I can say is: KEEP WORKING WITH YOUR SON AND NEVER GIVE UP! Things WILL get better. Insist that your school district help you. (By the way, is he in Early Childhood classes? He definitely should be. You can get a lot of help from the special ed counselor if you insist on it and don't take "no" for an answer.)
Hang in there, L.. God gives these special little ones to only the most capable Moms. If you don't feel capable right now, just keep working at it. You will be. Feel free to email me at any time.
S.