Why are Paul's letters the source of fundamental truths for the Church Age?
Paul is designated Apostle of the Gentiles for this age of grace, the Church Age. Therefore the Pauline letters provide the fundamental truths for salvation, the Christian walk, and operation of the local church. All such truths must come from Paul's letters.
Les teaches that during this age of grace, which is called the Church Age, Paul is the designated Apostle of the Gentiles. Consequently, Les feels the Pauline letters are where fundamental truths must be garnered for salvation, for the Christian walk, and for the operation of the local church. He stresses that it all has to come from these letters of Paul. This never implies the rest of the Bible is inappropriate, because Paul himself writes that all Scripture is profitable, inspired of God, and profitable for reproof and instruction in righteousness. Romans especially contains much that is fundamental for Gentiles, and this is the reason Les teaches as he does.