It ranks where it is for one simple reason. Google sees it as the authoritative site for that search term. Its not his page titles, its not his content, its not his domain. Its also not his backlinks nor his "insert secret unknown ancient chinese seo secret here". Its more of a combination of all the above.
Its the whole package and how its displayed to the web. He has more "green flags" on his site then "red flags" and definitely more then the other pages in that serp. Changing his theme, would most likely do little to his rank at this point.
Now you definitely do want to use a "friendly" theme, but in all honesty its
more often the webmasters who are seo friendly then their designs. And because its wordpress you can easily combine chunks of themes and actions from other themes. Just tweak your theme (whichever you chose) to do what you wish it to do.
If you need to find out what he specifically did with his wp install that differs from you, do a site:
www.site.com search in google, see what gets spidered, whats noindexed and how and wear he pushes the link weight.