Definately a naturopath and get a test for it, it's urine test and shouldn't cost too much.
He shouldn't come off prozac without having the alternative treatment first, and being on prozac whilst on the treatment shouldn't cause a problem. He'll have to come off prozac slowly when he does because of withdrawal symptoms, do it under the guidance of a dr.
It takes a while to see the effects. Does he get easily stressed and unable to calm down for ages?
When you get a naturopath find out about adrenal fatigue, histapenia and histadelia, and other things like candida as these can all cause depression! You can check these on diagnose-me website too.