Yes. There is no requirement that you live here. It is a multiple entry visa. You will be stamped in for 1 year each time you enter. Enter near the end of your first year and you will get another year.
Unless he can simultaneously meet the monthly income method and decides to use that instead when he applies for the next extension. You can choose either method at any time provided you can meet the yearly requirements when you apply.
The monthly income method definitely requires the funds to be identifiably deposited from a foreign source. However, they don't inquire as to the origin of the funds for the banked money method. Just show you have the 800,000 baht in a Thai bank for the requisite period of time.
Also, it depends on what Immigration office you are reporting to. CW in Bangkok doesn't ask to see a bankbook at any time until you apply for an extension.