First and foremost be a good friend or co-worker but don’t try to push the issue too much or you are going to turn him off. When I am at work, the last thing I would want is someone trying to convert me. He knows you are a believer through your previous casual conversation, let your prayers to the Holy Spirit have time to come to fruition. But set a good example at work, by not gossiping about others, doing a good job at work and helping others. This will show your friend a good example of how a Catholic Christian lives their life and is the best way to proclaim the Gospel.