Psychology and Free Will

I've been discussing free will online with a Christian university student who happens to be a psychology major. He believes in free will, and I not only don't believe in free will, but I also don't understand how a psychology major, of all people, could. This is what I wrote to him a few minutes ago:

If I'm not mistaken, you're a psychology major. Now what is psychology if not the scientific study of behavior and mentation, and how can there be a science of uncaused behavior and mentation? And if you say that the cause of human behavior and mentation, including human will, is the individual person, I say that it is unscientific to assume that the proverbial buck stops with the conscious choices of the individual without taking into account the biopsychosocial factors, many of them undoubtedly unconscious, that cause those conscious choices. In other words, it seems to me that modern psychology implies determinism. Thus, if you really believe in free will, it seems to me that you should render serious consideration to changing your major.