Can they charge my boyfriend for battery if there are witnesses but he did not really hurt me?

Obviously, they can because they did. If they have no evidence for probable cause, they should not have arrested him. If all witnesses told the police he did not hit you, then they had no probable cause.

They can charge him with anything. The key is proof. Why would he be arrested and why would there be witnesses if he did nothing?

When someone calls 911 and reports a domestic violence case (here in Washington state), the responding officer is supposed to make a decision based upon the facts reported to him or her and is supposed to remove the offending person from the premises and make an arrest if he or she believes that a crime has been committed. Many times over the years, women have called 911 out of desperation, and then when the boyfriend or husband has been arrested, the wife or girlfriend has second thoughts and wants to deny that there ever was an assault. Prosecutors have grown tired of the nonsense and will bring charges and subpoena witnesses to prove the case if they think they can get a conviction.

Yes, and you are foolish to stay in an abusive relationship.

They can charge him if there are credible independent witnesses. However, you are the alleged victim and if you are claiming that nothing happened , it will be up to the prosecutor to see if they want to pursue the charge.