How would a single person establish a common language with group that does not speak any language that this person knows? This group is from copper age period and modern man does not have any modern tech with him when he meets this group. To be specific, Modern Man or for short, MM cannot just call help or wait help to come. The surroundings are untouched forest.
Humans have the innate ability to learn languages. This ability is greatest in very young children, but it never goes away completely. During the millenia very very many people learned foreign languages by direct immersion or close contact. For a well-known example, the Iberians, Celtiberians, Gauls, Dacians etc. all learned Latin and eventually their descendants forgot their original languages and now speak badly broken forms of Latin known as Portuguese, or Spanish, or French or Romanian: do you think that the Romans set up schools to teach them Latin? No they didn't: they relied on the innate ability of humans to learn new languages. For another well-known example, most people of Central and South America speak Spanish or Portuguese -- and their ancestors must have learned those languages the hard way.
Once the stranded modern man manages to convince his captors not to kill him he can rely on the human urge to communicate. Basically, you point to a rock and say "rock"; he points to the rock and says "petra". You point to a tree and say "tree"; he says "arbor". You soon learn how to say "yes" and "no", "come here" and "go away" etc. After some time, you can carry on small conversations. After some more time you begin to feel at home in the new language. You may even become a poet: in his Letters from the Black Sea, the Roman poet Ovid who had been exiled to Tomis (modern Constanța in Romania) writes that he had written a poem in the Getic tongue and that "achieved the name of a poet among the uncivilized Getae".
For a scientific and practical approach see Kenneth L. Pike's monolingual elicitation techniques as discussed in "The Nature of Field Work in a Monolingual Setting" by Adam Makkai, or Language learning in the field by Mark Liberman (posted on Language Log, an excellent resource for those interested in all things linguistic), or the chapter "Monolingual fieldwork" in Jørgen Rischel's "Dilemmas and Paradoxes in Linguistic Fieldwork".