It's true that a limited diet can be a sign of autism, but not necessarily. It can also be a sign of a child who feels 'out of control' of other aspects of their life, so controls the one that they can.
But some kids do just seem to do this.
We have a young relative whose similar insistence has continued into his late 30s. I realised when he was young, that if I was to be a help to the family, I simply had to accept this. I didn't encourage it, or comment on it (except when necessary, such as checking he had packed his own picnic).
He is physically healthy, holds down a good job and has a girlfriend, although they appear to be a couple who enjoy each others' company rather than a serious relationship.
I can't say that he seems to me to be as joyful and purposeful as many of the other young men in our family. But he is happier than many, and that is reason to be glad.