The Catholic Church does allow Natural Family Planning, what the moral teachings of the Catholic Church does oppose is artifical Birth Control. Based on the constant teachings of the Church from the time of the Apostles forward, artifical means of preventing God’s will with regard to pregnancy are contrary to the Divine Plan. If your son is truly interested in becoming Catholic, his wife should also learn about what the Catholic Church teaches, so she can know what her husband is committing to, and understand why he is converting.
NFP supports periods of abstinance, and promotes healthy expressions of sexual intimacy as part of the love shared between two married people. Artificial Birth control has been promoted to abandon sexual intimacy for free love, and to avoid consequenses of behavior that would have been rejected by previous generations as simply base and not promoting love and intimacy.
[url:1qytdrth]http://nfpandmore.org/[/url:1qytdrth] for more information on Natural Family Planning and the Catholic position on intimacy and love.