In regards to authority!
If he believes that it is the bible alone that is authorative, than does he ever read commentaries, or listen to messages? Why?
If he does read commentaries, then why not some from guys in the first few hundred years of Christianity? Like Ignatius of Antioch on the Eucharist around A.D 105 or St. Cyprian of Carthage on infant Baptism around A.D 240.
If the word of God is the authority, then why all the preaching and teaching, are the words of scripture insufficient?
Why then, do men feel they need to explain the bible, if the word of God alone is sufficient, why not just read scripture aloud!
Are they not trusting that the words alone of scripture being read, will bring about conversion?
Since he is Baptist, does he hold to a confession of faith like the Baptist confession of 1689?
If there is no authority outside of scripture, then what does he make of what scripture says about authority?
How was the bible compiled, through what medium?
Is the canon of scripture closed? If so, by who’s authority?
Sometimes, you can get people to listen to other non-Catholics, that hold more Catholic views. For instance, their is a debate between John MacArthur, and R.C Sproul on Baptism using scripture alone.
R.C Sproul holds to a pro infant baptism view, and lays his case out pretty well, using scripture alone!
Both of these men are often on evangelical radio dials, and are well respected, so he may not listen to you, or any catholic apologist, but he may listen to R.C Sproul
You can find the debate on YouTube!
There are non-Catholics that have similar views to Catholic doctrine on Justification as well, guys like Charles Hodge, or Norman Geisler!
I find that many non-Catholics will steer clear of anything Catholic, but will read stuff by non Catholics (excluding Lutherans, and Anglicans, they stay clear from them as well, too catholic I guess :shrug:)
Hope that helps!