Chris Conti's Answer:
It is a very rare case indeed where a US based employer will give an assurance to an executive concerning the term of the employment. It does occur in some industries, media and entertainment comes to mind, but generally there are no "for term" contracts. Instead people are hired "at will."
Many times, because of this reality, we see candidates who are reluctant to relocate to a geography that could not support their next move if this current one did not work out.
For this person's situation, we would advise him to be up front with his hiring manager about the reasons for his interest in the role. He has clearly set forth his priorities and his new boss should understand these.
Given his level in the company, it is highly likely that his boss and peers have similar family related concerns. Moving kids who are still in high school is always a difficult situation. Perhaps the candidate can work with his boss to set up some 3-5 year goals for things they'd like to accomplish in this new assignment.
If he has a reasonable manager and shows progress toward those goals, he can in essence insure that his tenure will be at least 5 years long.
As a hedging strategy, it would be useful to get active in some local professional organizations as soon as he's situated in the new town.
Then if he senses things aren't working out as he would've liked, he already has a headstart in terms of local networking and relationship building.