Hi
For UK citizen he needs to apply for ESTA:
https://esta.cbp.dhs.gov/esta/esta.html?_flowExecutionKey=_cD6946CE9-6D2C-015C-03D1-DE3D76D090CE_k1DF26EC1-AF09-D706-6B20-9DF3B9725F04This is only needed every two years.
Also:
He will still be issued with an I-94W. If you cross by land it will probably still cost six bucks. By plane it's free.
Also VWP nationals resident in Mexico, Canada or adjacent islands are generally exempted from requirements to show onward travel to other foreign destinations. (Extracted from this link
http://travel.state.gov/visa/temp/without/without_1990.html around 30% of the way down - this section:
Which travelers may use the Visa Waiver Program to enter the United States?)