My kitten has round pupils?

Hes really scrawny and timid and his meow is more like a squeak with a kind of purr. Hes auburn, black, and white. His sister is a calico with slitted eyes and healthier looking. They both eat the same amount. Could something be wrong with him. His eyes are always round, not just when hes scared or its dark. He was alot easier to tame than his sister too and he cant jump as high.