If he is truly a Christian, and if these are actually demons, then he's going about this the wrong way. I don't have my Bible right here, so not sure of the exact references. One is coming from Jude. And, when the devil contented w/Michael, the archangel over the body of Moses, he did not rebuke him. Instead, he said, the Lord rebuke you. Then in another place in the Bible, it states that the devil walks around as a roaring lion, seeking whom he may devour. Submit yourself, therefore to God, resist the devil, and he will flee from you.
You cannot, on your own, resist either the devil or demons. It is only by submitting yourself to God that you can have the strength to resist because greater is He that is in you than he that is in the world. Next time he gets these visitations, instead of fighting w/them, tell him to start praying, quoting Bible verses, even singing praise and worship songs.
And, if he is truly a Christian, he cannot ever be separated from God. Christians are held in Christ's hands, the Father's hands, and sealed w/the Spirit. There is no way that they will ever let go what is truly theirs.