1. It is more natural to say "He is addicted to winning". 2. "He's learning a lot working here". Present continuous sounds more natural from those choices. Though I would personally use the present perfect tense here: "He has learned a lot working here". "The present perfect tense is one of the common verb tenses in English, used to show an action that happened in the past that is directly related to the present, such as actions that are still continuing or that indicate a change over time." Furthermore a preposition is not necessary in this sentence:)