2) Will he need a visitor's visa to enter the U.K and stay for a week?
NO
"As an American citizen traveling to the UK, you need a valid passport, a round-trip ticket and spending money! A visa is not required. You can apply for your application form on-line at the US State Department. "
Source:
http://www1.visitbritain.com/VB3-en-...passports.aspx