Here are some articles and other resources from Enjoy the Bible Ministries and written by Keith Blades that may help explain what we mean by "sonship".
"In Romans 6–8 the apostle Paul teaches us about our sanctified position in Christ and how God is dealing with us today under grace and not under the law. As he does so he declares that in accordance with this God has adopted us."
"Though we certainly are "of the household of God," and we certainly do have a familial relationship to God, "Abba, Father" is expressive of something much more than that."
...at Prayer in General, and at Sonship Prayer in Particular. "...the intercessory ministry of the Spirit of God within us is something that should be especially comforting and encouraging to us. For it concerns the issue of our prayer life as God's "sons" in this present dispensation of His grace, and it teaches us about the special provision that the Spirit makes for us when we as 'sons'..."
Enjoy the Bible Ministries also produces a number of DVD's and CD's related to the topic of Sonship as well as a number of other Bible study subjects.