A trendy boy, not to be confused with a pretty boy / metro-sexual, is a guy (or young male rather) who is well into modern fashion, looks or style. A trendy boy follows the previous (though still in style) /latest trends and fads of the moment.
Contrary to popular belief, pretty boys do not spend most of their time on their appearance, simply because their appearance is an attribute. Men with soft features, like oval faces, long / curly eyelashes, refined hair (e.g. curls, waves), captivating eyes, radiant skin, etc are generally considered pretty boys. Therefore, men who are over-concerned with their appearance and strive to make themselves appear "pretty" are labeled a metro-sexual and / or narcissist.
However, none of the aforementioned has to do with trendy boys. Examples of trendy boys are boys wearing the latest Jordan (or whomever) shoe; designer label brand shoes, braids (or dreads) / haircuts with designs / lines in them, latest electronics (e.g. cellphones, iPod, etc.), latest clothes and accessories (e.g. clear lenses, "Dookie" Rope chains; gaudy jewelery, Urban Outfitters / Forever 21 / Hot Topic / American Eagle Outfitters, etc. apparel), latest styles (like the "marching band / military" jacket look) ... You get the picture.