First, you must pray for him - that the Holy Spirit illumine his heart. You will have no success without prayer first.
GregoryL was right: this man has no interest in seeking the truth. He has simply found bitterness and deception that he agrees with. The demon sows the seeds of division, while the Holy Spirit always leads to unity. Obviously, neither he nor his sources have any authority to interpret the bible. Have him explain this:
2 Peter 1:20 (King James Version) “Knowing this first, that no prophecy of the scripture is of any private interpretation.”
Also, if he is a Baptist, he likely does not believe that baptism has any effect on the soul. Have him explain what Peter meant:
1 Peter 3:20-21 (King James Version) "Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water. The like figure whereunto even baptism doth also now save us (not the putting away of the filth of the flesh, but the answer of a good conscience toward God,) by the resurrection of Jesus Christ:
And Paul:
Acts 22:16 (King James Version) And now why tarriest thou? arise, and be baptized, and wash away thy sins, calling on the name of the Lord.
The demon specializes in shotgun style attacks, using numerous (62 for example) errors. Try to get this man to focus on one or two - always bringing him back to those when he wanders. Don’t expect him to huff and puff too long. He will probably cut off communication, since he cannot refute the truth. In the end, remember that nothing that Saint Monica said to Augustine had any effect. Her years of prayer are what turned his heart.
Christ’s peace.