Two points here really:
1 there is no incontrovertible proof that he claimed a unique sonship
I dont know what he means by incontrovertible, but John 14:1-4 1Do not let your hearts be troubled. Trust in God; trust also in me. 2In my Father's house are many rooms; if it were not so, I would have told you. I am going there to prepare a place for you. 3And if I go and prepare a place for you, I will come back and take you to be with me that you also may be where I am. 4You know the way to the place where I am going. seems to answer nicely.
2 unique sonship not open to other persons
Besides being redundant any unique relationship is, by definition, not open to other persons this misses the Point. While Jesus is the only begotten Son of God, we are all sons and daughters of God.