Get him on the whole "Faith Alone" concept. That and "Bible Alone" are the two keys to the entire Protestant view of things. Once those are gone, everything else falls into place.
Catholic teaching: We are saved neither "by faith alone," as most Protestants believe, nor "by works." We are saved by grace alone -- a free gift of grace that we accept through faith and works, or, if you prefer, by a living, saving Faith -- a Faith that "worketh in love" (Galatians 5:6). Scriptural reasons why "Faith Alone" ("Sola Fide") is bogus:
" Not all who say "Lord, Lord" will enter Heaven: Matthew 7:21
" Many are called, few are chosen: Matthew 22:14
" Those who do good are resurrected to life: John 5:28-29
" God renders to us according to our works: Romans 2:6
" Those who die in mortal sin won't be saved (i.e., whether they've said the "Sinners Prayer" or not): 1 Corinthians 6:9
" Faith, Hope, Charity, and the greatest of these is Love: 1 Corinthians 13:13
" Works and Faith alone don't save, but grace by Faith that works in love: Galatians 5:6
" Work of Faith: 1 Thessalonians 1:3
" Not doing certain things is worse than being a heretic unbeliever: 1 Timothy 5:8
" Those rich in good works may lay hold of eternal life: 1 Timothy 6:17-19
" Those who hear but don't do the word are lost: James 1:21-27
" Faith without works is dead, even devils believe: James 2:14-26