One way of fostering buy-in for a market-driven approach is field trips. On top of the normal prospect visits he should be making, let the product manager bring developers to customer sites to observe and experience use (or non-use) of the product in real life. Better yet, have career days in which each developer actually sits in for a customer and plays her role for the day. You will have to get permission from customers, of course, but you have two strong arguments to convince them:
- Your team, for free, will do your customers' work for them.
- It ultimately benefits the customer for your developers to understand the user experience.
A product manager's role is to understand the market and communicate that understanding to the product team. To get buy-in from developers, it's not enough to throw market requirements documents at them. Field trips give developers the perspective they can get only from first-hand experience.