Explain the quote "'Jack' A taboo was evolving around that word, too."It's from Chapter 8.
The short answer is that the boys did not like using the name 'Jack' and are unconsciously referring to Jack as "him".
To extend your response, there are two key words in the question which need to be discussed: taboo and evolved.
Taboo--the strong social prohibition over certain actions, objects or words--is an extremely strong word. More than a law or rule, to break a taboo strikes at the heart of both society's order and psychological stability. An extended discussion may include reference to Totem and Taboo by Freud. To hold something in taboo suggests both fear and reverence. Taboos are often most strongly associated with primitive cultures and basic instincts
The word evolved is also extremely important: it is something grown organically, naturally. Evolution is also a survival mechanism: you evolve physically and emotionally in order to adapt to a changing dangerous environment around you (e.g., Darwinism). This may suggest that the taboo around saying Jack's name is a survival mechanism to cope with the way that Jack is changing the world around him (it reminds me of the prohibition on Voldemort's name in the Harry Potter books)
You need to contextualise the quotation too: it comes as a response to Ralph's question "What makes things break up as they do?" and at the mention of his name, he appears naked save for paint and a belt--like a demon summoned by its name.