Agree with Christie - it is a VERY long and difficult process to teach a child with food issues to eat a wider range of foods but it can be done. Much of it comes from sensory sensitivity and you need to be extremely patient and consistent if you're to teach a child to accept new foods. Christie's advice about doing it in small steps is excellent, starting with accepting a food on the plate then gradually progressing to touching, smelling, licking etc until they feel able to try eating it. This can take months if not years.
My ds (9, with AS) ate the same meal for four years - chipolata sausages (only from a particular butcher), smiley faces and beans. Using gradual techniques we slowly encouraged him to accept trying new foods, often with him sitting on my lap while I spoonfed him with a tiny amount of the new food, swiftly followed by a favoured food. It took months for him to accept something new but when he did we made a big fuss, gave praise and rewards, and encouraged him to continue trying. He now has a very good nutritional diet but still doesn't eat the same kinds of foods as NT children have - eg pizzas and the like (doesn't do combined foods apart from sandwiches, and that's a lengthy tale in itself).
Don't give up, it can be done but first establish a core range of foods that your child enjoys (however small) then gradually add new ones, but remember it will probably take a very long time.
Oh, and I found it essential to rotate newly introduced foods regularly, as if I left it too long between tastes he'd forget he'd had it and consequently we'd have to start over again with that particular food. Soul-destroying! And don't over-load by trying too many new foods at once. One at a time works best.