DO NOT enter long discussions without agreeing on a common ground, a mutual point of agreement. If you don't start from somewhere you both agree on, your conversation will dissolve into endless discussion, charge and countercharge, as he takes you on a merry chase through history, scripture, philosophy, and theology. Unless you agree on something, he can deny belief in whatever you affirm. DO face the question of authority. The issue is that his source of authority is opposed to the teaching of Holy Scripture.
Prayer can work in the sinner's heart where no amount of reason or common sense seems to penetrate.
Pray repeatedly for the one you talk to, for many of us have been saved as a result of the consistent prayers of others in our behalf. No limit can be set on the power of prayer. Cultists are seldom won on the first conversation or even on the tenth. One couple shared with a JW in their home every week for six months, and three years later, as a result of the seed first sown by the Christian couple, the young man turned his life over to Christ. There are times we will be seed-planters and someone else will see the harvest.