The man that Buck sees in his dream is described as follows:
This other man was shorter of leg and longer of arm, with muscles that were stringy and knotty rather than rounded and swelling. The hair of this man was long and matted, and his head slanted back under it from the eyes. He uttered strange sounds, and seemed very much afraid of the darkness, into which he peered continually, clutching in his hand, which hung midway between knee and foot, a stick with a heavy stone made fast to the end. He was all but naked, a ragged and fire-scorched skin hanging part way down his back. (chapter 4)
The physical description of this man makes it clear that he is intended to be an example of prehistoric man: hairy, practically naked, long-armed and short-legged, inarticulate, equipped only with a club. He is a symbol of the primitive, a figure from ancestral memory, a reminder of the time before civilisation. This is of course pertinent to the book as a whole as it details Buck's gradual reversion to primitive life in the cold Arctic regions where man and beast alike fight to survive in a harsh environment. This figure becomes more prominent in Buck's dreams as he becomes ever wilder and more wolf-like. In the end, absolutely no trace remains of his former civilised ways.
The reversion to the primitive, the reminder of the brute necessities and instincts lying behind the veneer of civilisation, was a favourite theme of London's, and other writers like Frank Norris who partook of the literary movement known as Naturalism around the turn of the twentieth century. Much influenced by the rise of evolutionary theory and related ideas about the persistence of primordial impulses and instincts in the modern world, the primitive strain often appears as savage and brutish in the work of the Naturalist writers. However, in The Call of the Wild it is also celebrated, in the form of Buck, the dog that rises to be a great and powerful leader.
The man is no particular person, but rather a representation of a more "primitive" type of man. He is frequently described as hairy; this implies a sort of caveman archetype. Buck is basically dreaming about the early humans who tamed wolves and kept them as pets. London makes several references throughout the book to "old instincts" resurfacing in Buck - it is, in a way, like Buck is reliving the experiences of his ancestors, and the more dangerous world in which they lived.