A trend-setter or trendsetter:
someone who starts a trend, or makes one more popular
The other answers on this page have a couple other fine suggestions — and a thesaurus lookup turns up more suggestions still —, but be aware that some of them require additional qualification. For example, it's typically "a harbinger of something", or "a precursor to something". So you won't be able to just say "he is a harbinger" and leave it at that; likewise, "he is a precursor" will only raise questions if no further context is supplied. "He is a trendsetter", on the other hand, is perfectly self-explanatory and self-sufficient.