I would change the first part of the second sentence from "He agrees with existentialists in some perspectives" to "However, Bellow agrees with certain perspectives of existentialism." First, the word "however" is used to show some exceptions to Bellow disagreeing with existentialism as a whole because he agrees with certain ideas (like "the alienated state of man in the modern society, the authentic existence man should seek in his life, and the responsibility man should take for his own way of existence"). Second, because you're describing existentialism as an abstract concept, stating that Bellow agrees with existentialists (a concrete idea of people who have existentialist views) doesn't really make sense.