Vocation and Call or Will Work for Jesus?

This afternoon one of my closest friends called me to talk about his vocation. He is in a Science department at the great evil also known as Auburn University. He has a great mind, great personality and a great desire to search for God. He loves to talk about issues of faith and theology as well.
He has a problem though; he is having trouble deciding if he should stay in his field. It is not because he does not love science or that he is not good at what he does, the problem he is having stems from guilt ingrained in him from his youth. It tells him if he has a love for God and feels a call to God’s service then he can only serve God in some form of formal pronounced ministry. I want to help him understand that it does not matter so much what his vocation is he can still think about theology, pursue knowledge of God and practice and explore the faith.
I have suggested, much to the chagrin of some who read this blog, Parker Palmer’s book Let Your Life Speak. I have two questions: 1. Does anyone have any suggestions on what to tell him to help him think about all of this more? 2. Any other general thoughts about vocation and what God’s call looks like in many different fields of study?

Pax
Stephen