Fathers should teach and guide their sons, but they should also be there for them. David’s biological father is nearly worthless, but Uncle Axel is a good father figure for him.
David’s father is harsh and cruel. He desperately wants to keep control, to the point of severely beating David on more than one occasion. We know this because of David’s reaction when the inspector comes to his house. David refuses to tell them about Sophie, and instead of protecting his son from the inspector, David’s father offers to beat it out of him instead.
I knew well enough what that meant, but I knew, too, that with my father in his present mood it would happen whether I told or not. I set my jaw, and turned to go. (ch 5)
There is little doubt that if David’s father knew he was telepathic he would turn him in. He reacted almost as severely when David accidentally said he wished he had three hands.
Uncle Axel, on the other hand, is a good father figure. When he finds out about David, he does not turn him in. He just tries to protect him, even to the point of killing Anne’s husband to prevent him from telling anyone.
I want you to promise that you will never, never tell anyone else what you have just told me -- never. It's very important: later on you'll understand better how important it is. (ch 4)
Uncle Axel is not even a “real relative,” but he seems to be the only one to actually look out for David and have his best interest at heart. Since he is David’s best friend among the adults, David listens to him and appreciates his advice.
David is fortunate to have Uncle Axel, because he is in real danger. Anyone, even his own father, would give him up if they knew he was telepathic.
As I cannot edit the question, I meant to ask about Uncle Axel and Joseph Storm as a father figure to David.