Perhaps he needs to be reminded of the old adage: Insanity is when you keep doing the same thing, but expect different results.
Does he get the kind of "results" that the WTS tells him he'll have? Magazine Placements™, Bible Studies™, new ones being recruited? The WTS wants him to believe that the reason he doesn't succeed is because of a flaw in himself, when in reality, the flaw is in the methodology. How many successful businesses utilize door-to-door sales anymore? The fact of the matter is that the Field Ministry™ is, by and large, an ineffective recruitment tool, just as ineffective as cold calling done by telemarketers.
If he wants to think of Field Ministry™ as a way to spend time with fellow believers and reinforce each other's delusion while cooped up in a crowded vehicle between territories or over coffee break, that's one thing. But if he's looking for a way to become more successful at it (as defined by the WTS) he's likely to become less motivated and less satisfied.
Perhaps he needs to redefine what "success" means when it comes to the Door-to-Door™ work, and have it be his definition, not theirs.