I'm a Chinese from Singapore. The practices I am familiar with might differ from the Chinese of other countries. As a quick background, as a Singaporean, I am heavily exposed to both western and eastern cultures. Some of us would go on to be very familiar with both. Most of us, however, will lean on either one side of the other. I lean towards western, but I still retain some eastern heritage. Just think of me as the next best thing until a true-blue Chinese-Chinese guy comes along.
Anyhoo, one thing about the 'Chinese' identity is that it is more... diffused than you might think. There are hundreds. Hundreds! Of dialect groups. True, they were all forced to use a common writing script ever since thousands of years ago, but they, at least, get to keep their dialects. Each dialect has a different way of pronouncing a word, and some would have entirely different pronunciations of a mandarin word altogether. This leads to different romanisations of said words, and therefore, names.
A name would further be subjected to differentiation by subgroups within a dialect, and by 'personalisation'. Some people just want to spell their name differently, or just didn't know better than the version they just cooked up before a government clerk. You can get some weird results, so Bahn and Banh or Binh would be just as realistic as Ban. However, from my Singaporean Chinese culture, I don't know when's the last time I've seen an h being slotted into a character hanyu pinyin..
One thing to note though. Most of the Chinese first names I have encountered have two characters. It's rare to encounter one with just one or three, unless they're nicknames or shorthands. I hope you know that..?
Otherwise, if you honestly didn't know, a simple fix would be simply to call him Ah Ban/Bahn/Banh/Binh. The 'Ah' is a very common prefix, though it's been associated with being simple or from the country side. A hick-name if you will. In this day and age though, fewer people care. Alternatively, it is part of a term of endearment. I got a guy at the office who everyone calls 'Ah Tee'. He's like the office boy here, lol.
EDIT: Oh man, I got way ahead of myself. Other than inserting a 'h', I can only think of Buan. Slightly different pronunciation, sure, but it's the only realistic one I can think of. You could always go with Baan, but that would really be stretching it - I'm questioning my memory right now, as I'm not sure if I've seen a double A before in Chinese name romanisation. Otherwise... Phuan, which is a slight modification of Pan that I might have seen somewhere. Note that pronunciation in Chinese is very different from pronunciation in English.
For a quick guide on Chinese pronunciation, look up hanyu pinyin.