Did he tell you that the Bible is what God says? All of it? Every word is right?
Yes and He did it from 2 Tim 3:16.
I can't find the post where you mentioned 2 Pet 2:21 and Heb 6:6. I know there was at least 1 more but I don't remember what it was. So I will reply to the ones I remember.
First, there are many simple and clear verses that say we can't lose our salvation. I gave you 2, but there are many more. I will give you one more because I always start my study with a general rule---don't use a difficult verse to refute a simple verse.
I Pet 1:23 - For you have been
born again, not of seed which is perishable, but imperishable, that is through the living and enduring word of God.
One only needs to know the definition of "imperishable" to understand what is being taught.
2 Pet 2:21-22
In context, starting in verse one, this whole chapter is not abut salvation, it is about false prophets---those who tell other what God told them but He did not. The way of Balaam is his thinking if Israel would commit great sins, God would curse them and he offered his services to a pagan king for money. He prostituted his office.
It would have been better if he has not known the way of righteousness than to know it and ignore it. God may take "I didn't know" as an acceptable excuse and grant them His grace.
Heb 6:6
This is about salvation and it is written about Christians. The context starts in 5:12. Certainly Christians can fall away from their faith. then it is impossible for man to renew them, but to think an omnipotent God who can speak the universe into being, can't or wont renew someone He loves is foolishness. Their God is to small and lacking in compassion.
Mt 19:26 - And looking at them Jesus said, "with people, this is impossible, but with God all things are possible.
Let me give you an allegory that teaches the same thing. Being born again, teaches us a spiritual truth using a literal truth we all understand. We play the same part in our spiritual birth that we played in our physical birth---nothing.
My son, good, bad or indifferent, will always be my son. That is a biological fact that can't be changed---his conduct can't affect his relationship to me. It can only affect our fellowship, and that can be restored even among humans.
If you want me to address the other(s) you mentioned, mention them again when you repy to his post.