Do you think Buck from Jack London's The Call of the Wild would be able to rejoin man at some point in his future?

I would say Buck would be unlikely to rejoin man, but that doesn’t mean he never could. Buck’s early experiences with humans were good. He was the judge’s dog, and he was the king of the house. He had a very good life. Buck then had some terrible experiences; he was abused and neglected by the sled drivers. Finally, Buck had a good life with John Thornton, the first sled driver to show him love. Buck went into the wild after Thornton’s death, and the ending of the book seems to indicate he stayed there.

The years were not many when the Yeehats noted a change in the breed of timber wolves   . . . But more remarkable than this, the Yeehats tell of a Ghost Dog that runs at the head of the pack. They are afraid of this Ghost Dog, for it has cunning greater than they, stealing from their camps in fierce winters, robbing their traps, slaying their dogs, and defying their bravest hunters (Chapter 7).

Buck would not feel kindly toward the Yeehats because of John Thornton, whom he loved. Buck would, however, also feel at home with the wolves. He was the head of the pack and created his own hybrid wolf-dog tribe.
This does not mean that a scenario could not come into play where Buck would not return to humans. In White Fang, there is a severe drought that drives the wolves and narrators to distraction. The titular White Fang is a hybrid, and his story is the opposite of Buck’s in some ways. White Fang starts off wild and becomes domesticated. 
Circumstances change. Buck's experiences certainly seem to indicate that. It is possible to imagine a circumstance where Buck would lose his tribe and be vulnerable enough where he would need people again, even though as the book ends he seems to be in a strong position.