Well, there is no simple solution unfortunately, because of the irregular patterns of the singularization – pluralization forms of english language. In many cases adding/removing the “s” from the word ending is not enough.
I've done some research, and looks like there might be a pre-created code that covers most of these cases as well. I've made a few changes to integrate it with the search, and it looks like it's working as a temporary solution.
Try putting the code on this url to your theme's functions.php file: http://pastebin.com/raw.php?i=m4q0VF6K
This will go through all search words inputted and detects if in singular/plural form, then it will append (invisibly to the user) the opposing forms as well to the search phrase.Best,
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