Whole word search in less?

Running less --version gives

less 487 (POSIX regular expressions)

so you need to use POSIX regexps in less. To learn more about them you can read through re_format(7) to find

There are two special cases= of bracket expressions: the bracket expressions `[[:<:]]'
and `[[:>:]]' match the null string at the beginning and end of a word respectively.

So to search for word you need to enter [[:<:]]word[[:>:]] on the search prompt (which probably isn't very useful in most situations).

PS: This also applies to the version of less installed via Homebrew. You might have more luck by compiling it yourself and specifying an alternative regexp library (or by modifying the source accordingly).