Witherington points out that Jesus prayed to God using the term "Abba", which is a term of endearment. This, he says, shows that Jesus thought of himself as the "son" of God as distinct from the prevalent usage where kings were also considered "sons" of God. But Witherington immediately undermines this proposition by stating that Jesus also taught his disciples to pray to God using the term "Abba". So this term does not, after all, denote a special exclusive relationship of the kind usually claimed for Jesus.
Added to which, the chapter doesn't address the issue of reliability that's inevitably triggered by a passage such as this:
There can be no doubt however, that Jesus did not view His relationship to God as simply identical to the relationship King David had with God. For one thing, it tells us a lot about Jesus that He prayed to God as Abba which is the Aramaic term of endearment which means dearest Father (see Mark 14:36, Abba is not slang, it does not mean "Daddy.")
One of the big mistakes in Christian apologetics is just focusing on what Jesus publicly claimed to be.