That's an interesting question.  Does he happen to have a history of building and selling other houses after "deciding not to live there"?  If not, there are some regulations under Section 1250 that might help, but this isn't Section 1250 property -- nothing there to depreciate.  I found this online but couldn't find its source:

Holding period. Use the following guidelines for figuring the applicable percentage for property with two or more elements.

• The holding period of a separate element placed in service before the entire section 1250 property is finished starts on the first day of the month that the separate element is placed in service.
• The holding period for each separate improvement qualifying as a separate element starts on the day after the improvement is acquired or, for improvements constructed, reconstructed, or erected, the first day of the month that the improvement is placed in service.
• The holding period for each improvement not qualifying as a separate element takes the holding period of the basic property.

https://www.biggerpockets.com/forums/51/topics/256349-tax-pro-question-one-year-clock-on-new-home-bu...

If you have to treat it as two separate sales, I'm not sure how you would allocate the proceeds.  Initial cost basis, or current FMV (as suggested by @TaxGuyBill )?  I would want to find a way to justify stating that the cost basis and FMV of a house that just cost $200K to build is the same amount -- $200K.  So all the gain is from the land, and long term.  

Maybe you should call IRS and ask their Holding Period specialist.  (<---Inside joke.)