What I recommended above are good for people just starting out.
I would say get him a disposable if you do not want to spend a lot of money. He can use it as he wants and just put it in his pocket and use it again later. He doesn't have to use it everyday and it is will still work. It shouldn't go stale or anything.
I have had some disposables that sat around for weeks and I used them no problem. They are basically good until the liquid inside runs out. Probably around 200 puffs give or take depending on how deep a puff he takes.
With the refillable like I linked you above you buy cartridges, a pack of 5 is $10 http://www.halocigs.com/turkish-tobacco-cartomizers.html
Each cartridge equals roughly 1 disposable so these are absolutely cheaper than the disposables.
When you go with a tank system like I showed you above you buy the eliquid and fill the tank yourself. This is more intermediate and I would not suggest you go that route for him. If he finds he likes e-cigs then that is what I would recommend for the upgrade.
So in conclusion each cartridge is about the same as 1 disposable, it is way more cost effective in the long run.
The triton tank system is excellent but a beginner may find filling the tanks a pain and not want to bother. You can get eliquid from many places and for very cheap. I buy most of my eliquid from here:http://www.mtbakervapor.com/electronic-cigarette-nicotine-juice/
They have a large selection of different flavors and you can get a 30ML bottle for about $7.50 . 30 ML would last him if he only uses it every couple of weeks a very long time. The triton tanks hold 2.4ML of eliquid and that amount would last him using it for probably 1 beer drinking night depending on how much he puffs it. If I go out to the pub for the night I bring a full triton tank and have never run out.
Also the triton battery will last much longer than the G6 as it is a lot bigger. The G6 would probably need to be charged every couple of hours of use while the triton you can get 5+ hours of heavy use before it needs recharging.
I can't really give you exact numbers because a lot of this depends on his personal preference such as how much he puffs it , how long his puffs are etc...
All that said, get him a quality disposable first just to get him interested in the idea so he can see how good it is. Then go from there. Some people just don't like e-cigs or he may really not want to give up real ones so no reason you should spend a lot of money up front.
Many people give their friends disposables to get them to think about switching over. It is a good gateway into the world of e-cigs.