At 20, its going to be difficult. He's got the independence and rights of adult, although few 20 year olds have the wisdom to use that responsibly (I know I didn't!).
Realistically, you can't expect to convince a 20 year old to stop doing something they've decided to do. I think the best you can work towards is making sure he knows all of the downsides.
You've already mentioned the health aspect, and that's certainly huge, but in my experience, most 20 year olds operate under the assumption that they are effectively immortal.
Some other effects of smoking that he may not have fully considered, and may strike a little bit closer to what he considers near and dear, are financial and social problems.
Point out just how much money he'll spend on cigarettes over the course of a year. Many smokers find themselves smoking a pack a day, or more, and this adds up to some staggering amounts invested over time. If your colleague's father smokes a lot, you could find out how much he spends a year, and multiply that by however many years he's been smoking.
The other aspect to point out is social. This is a bit of a double-edged sword, because peer pressure is the biggest reason most people start smoking, so it may be that everyone he hangs out with smokes. However, I've never met a smoker who objected to my not smoking, even when I was trying to quit. More importantly, though, smoking is a big turn-off for many people, and if he finds out that potential romantic interests may immediately blow him off because they don't want to "kiss an ashtray", it may help put things in perspective. Mouthwash only helps so much; there are quite a few non-smokers out there who simply have no interest in long-term relationships with someone who has to gargle before makeout sessions in order to not smell/taste offensive.