Ranay is right. Many children will want to eat the same thing every day. The child finds comfort knowing what to expect.
If you want o encourage a variety, each week when you go to the store, just ask him if he would like a different kind of sandwich. Make a few suggestions, but do not push. At some point I remember telling my daughter, if you ever want something different in your lunch, just let me know. At some point she asked for a green salad with chicken pieces. Yes this was in kindergarten. Then in first grade every once in a while she would ask for a plain turkey sandwich. I would switch out the piece of fruit from an apple to an orange. She loved dry fruit, apples, apricots, mangos. One time when I had run out of bread, I gave her some pretzel sticks with a small container of peanut butter for her to dip, she liked it, so every once in a while I would do this as a treat.
Tortillas, pita bread and crackers can be exchanged for the bread. Cheese sticks hold up well since kindergartners eat so early in the day. Cold pizza, if you child likes it is one of the favorites at the elementary level.