With the .cn domain name, would it be possible if he was to purchase that domain name and then geo-target that in google webmaster tools, and when link building, and then look into developing the new chinese site later on
I can't see how that would help all that much. If he has no Chinese-oriented content, there's no point in link building or geo-targeting. My advice would be to register the .cn name now, then develop the content, and then promote the site though link-building, etc.
Is the client able to develop content in Chinese? I would have thought that was an essential pre-requisite for this exercise (but keep in mind I know nothing about the market for antique clocks in China or elsewhere). And it's not just a question of language; it's also explaining the characteristics of the clocks, and the method of purchasing them, in terms that are meaningful to a Chinese customer.
In fact, the more I think about this, the more I see that the real question isn't how to SEO the website: it's how to sell the clocks in China. Has he considered, for example, looking for a Chinese distributor or wholesaler to work with? (That might be a stupid question; obviously, you don't sell antiques wholesale [do you?]; what I'm trying to say is that there's more to doing business in a given country than targeting your website at it.)
As for whether the domain should be .cn or cn.com, I'm not sure what the difference is. Is .cn.com a real TLD, or are you suggesting he should register a .com which happens to have .cn as the last three characters of the actual domain? If so, I would advise against that. For SEO purposes, a .cn would be far better. It's possible he would need to register it through an agent who is physically in China, but that shouldn't be too hard to arrange.
I hope this is of some help. It's an interesting question, and I hope your client does will with his idea.