Why did Abraham grow impatient and use a slave girl?
God's promises to Abraham were continually repeated but took years to be consummated, so he grew impatient. Since he could not produce a nation of people without at least one son, he succumbed to the temptation to use a slave girl. That union produced Ishmael, as the context explains.
Les explains that all through Abraham's life God kept promising, but some things took years before they were consummated. That delay is why the poor man grew impatient and finally succumbed to the temptation to use a slave girl. After all, he could not have a nation of people if he did not have even one son. This gave rise to the birth of Ishmael, which Les says will be examined in a future program. The broader context shows these promises to Abraham are the basis of Gentile privilege and that Israel will yet be grafted back in, as Paul says in Romans chapter 11.