Himself is correct and a reflexive pronoun in English. Himselves is incorrect and should not be used in English.Him self is also incorrect.
He needs to bake the cake himself.
Himself is the reflexive pronoun for the pronoun “he”. We use a pronoun in place of a noun when we don’t want to state the noun. A reflexive pronoun is when the pronoun is both the subject and the object of the sentence. This happens for example when the person(or people) is both the doer and receiver of an action.
He has to cut the grass himself. We had no gardener.
Himselves is not correct in English. The pronoun “he” is always singular in English.
“Him self” is not used as a reflexive pronoun but “his self” can be used in another context. “Self” is a noun that means a person’s essential being. In this case, you can use “His self”
His self has many layers.
“Hisself” is a non-standard form of “himself”. It is used primarily in the southern states of the United states.