I found many replies in this thread misleading...... In Taiwan, if he's your personal friend, calling him by his Chinese name would work best. If a business friend, then "陳先生" would sound best. If you say "陳同志", I believe you will definitely get your ass kicked (and even much worse), as the word "同志" sounds so communist that people in Taiwan will feel extremely uncomfortable and might think you're playing with them. On the other hand, "小陳" sounds too Mainland Chinese, which is also not frequently used in Taiwan. "陳哥" works only when he is more than just an acquaintance of you (and he'd better be older than you or at least at same age, or it would sound very weird), which isn't really applicable in the case you mentioned. "陳同學" wouldn't work unless he's a student.
My opinion: stick with "陳先生" or his Chinese name. Another way that works very well in Taiwan is to call him by his "English nickname", as most Taiwanese people would have one.
Last but not least, "陳" is officially romanized as "Chen" in Taiwan. Romanization in Taiwan is really inconsistent and several different systems co-exist, which I know it's a great pain for foreigners (and even for us). However, that is mainly because we never use romanization system to learn Chinese since we were at school. In Taiwan, the system we use to teach kids Chinese is called "注音" (zhuyin), which consists of a bunch of symbols like ㄅㄆㄇㄈㄉㄊㄋㄌ. I don't want to say too much beyond the original topic of this thread. Feel free to look up the wiki page if you're interested.