I think he is the pioneer of this concept. He set the ground for a more cognitive research in different countries and as a result in a different spatial perspective.

He didn't invent it. Agglomerations is the cognate term in economics and it goes back to Alfred Marshall in 1890. But Porter did popularize it and developed a business strategy framework that made it more applicable to firm business situations as opposed to general economic development.  If you want the latest economics version see Paul Krugman and the New Economic Geography. 

Cameron is right, Porter clarified the business strategic rationale for why cluster conditions are desired by firms of various types, which augmented the previous economic rationale for their support in creating local economic vitality.

In my view with the work of Porter a switch from competitiveness of nations to regional clusters occured . So initially, localized assets and relations were not that much important in the conceptual framework provided. The concept then gained momentum within a broader reconceptualization of the spatial conditions for innovativeness, in line with other concepts like regional innovation systems, creative milieus and so on, with third Italy, automotive clusters in Baden-Württemberg, Silicon Valley and so on as prominent examples and parallel to regioanlisation dynamics in the European Union. One important reason, as outlined by Lagendijk/Conford (Regional institutions and knowledge-tracking new forms of regional development policy, Geoforum 31, 209-218)  were the activities of Porter with his agency Monitor, were his broad ideas on competitiveness and the notion of initially more or less non-spatial clusters was embedded into regional development concepts. So Porter was not just a provider of a valuable concept, but a stakeholder in diffusing and utilizing the approach.

Poter's contribution consist on renewing the concept of agglomeration by producing empirical evidence.