Hello! All these sentences are correct and you can use them interchangeably, but according to English grammar the "will" form expresses a more uncertain future, so the person speaking does not know whether he will be back or not, nor when he will be back. The "-ing" form, on the other hand, expresses a certainty that he will be back sometime, even if the speaker does not know when exactly. The "-ing" form is more informal, and more used in everyday communication.