[This is a difficult argument to have without admitting that he's an alien from an advanced race whose members have occasionally passed themselves off as gods. He can't use all his favorite human insults.]

Science isn't just a body of knowledge, it's the method by which new knowledge is created. When you encounter the mysterious and the unknown is when you need science most.

To claim that something is a god is to dismiss it as inexplicable and beyond the laws of the universe. You prove otherwise the same way you prove anything. Subject it to scientific experimentation.