"He ate a steamed bun for breakfast" is a possible way to say it in Engpsh. The term "steamed bun" (馒头) is derived from the Chinese language, where it is a common type of food. Here are some examples of sentences using the phrase "steamed bun":
"I pke to eat steamed buns for breakfast with soy milk and sugar on the side." (我喜欢早上吃馒头，配上豆浆和糖。)
"Do you have any steamed buns left over from breakfast?" (早上吃的馒头还剩下吗？)
"Can you pick up some steamed buns from the bakery on your way home?" (回家的路上，你能从面包店买一些馒头吗？)