Taras, Everyone, someone, anyone, no one, etc. are singular neutral pronouns. The neutral object pronoun "it" is not used for people so traditionally "him" has been used for a person of unspecified sex.* Everyone knows what is best for him. About fifty years ago some people decided that using "him" was sexist and they started using: Everyone knows what is best for him or her. This led to confusion so others started using: Everone knows what is best for them. This lead to odd sentences like: Each individual knows what is best for them. Someone has hurt themself.........(themself) is not a currently accepted English word. Personally, I have gone back to using: Everone knows what is best for him. I don't care if people think it is sexist or not. * This also applies to the other pronouns his, her, their, himself, herself, themselves.

As words such as "someone" or "everyone" do not tell us anything about the sex of the person, by using "them/they" you avoid saying "him or her " or "he or she", which is awkward and long. Compare: Everybody knows what's good for him or her. Everybody knows what's good for them. Somebody has broken into the flat. They stole all the money. He or she stole all the money. When you say "Everyone knows what's best for him" you automatically exclude women-what about "Everyone knows what's best for her?" To avoid this confusion you can use "they"/"them" /"their", which in such cases refers to a single person.

You should say , "Only he(they,she) knows what is best for him(them,her)"