Since he believes that if one is “washed in the blood” they are forever saved, the question becomes, has he really been washed in the blood? Because if he had, then he would show evidence of that. An informed Protestant who believes in Eternal Security would doubt very seriously the salvation of this individual. “Work out your salvation with fear and trembling” indeed.
Once Saved Always Saved is a doctrine ingrained in many Evangelical Christians, and they quote many scriptures to “prove” it, so you probably won’t be able to convince him otherwise, although you may be able to plant a seed of doubt in his mind which may grow over time, and perhaps eventually he will come to the knowledge of the truth.
What should be of high importance to him, though, is his witness for Christ: that is, not only his words, but more importantly, his actions. Are his actions leading others to Christ or away from Him? Do they look at him and see the holiness of God?
When I was an Evangelical Christian, I was convinced of Once Saved Always Saved, but it was drummed into our heads that we had to be a good witness for Christ in our words and in our actions. I would hope his church teaches the same thing.
You might want to ask, in a curious manner, what his church teaches about righteous living. So many scriptures come to mind, which after some prayer, you might want to share with him. You mostly want to pray, though, that the Holy Spirit will convict him and point him in the right direction.
Here’s a couple of scriptures:
Titus 2:11&12 For the grace of God has appeared, bringing salvation to all men, instructing us to deny ungodliness and worldly desires and to live sensibly, righteously and godly in the present age.
Romans 13:13 Let us behave properly as in the daylight, not in carousing or drunkenness, not in sexual promiscuity and sensuality, not in strife and jealousy.
But, back for just a second to Once Saved Always Saved: even St. Paul was not convinced of his own salvation when he said in I Corinthians 9:27 I discipline my body and make it my slave, so that, after I have preached to others, I myself will not be disqualified.