Strawman argument. Atheists don't say that everything is physical. They say there are no gods. It is fallacious to say that atheists don't believe in concepts, ideas or logic. Making such arguments make Christians look very silly indeed.
It isn't a straw-man because it doesn't argue that the atheist doesn't believe in concepts, ideas, or logic. However, it does argue that, within his own worldview, he cannot account for why they exist.
The atheist typically believes in materialism, naturalism, or empiricism - all in varying amounts. Yet none of these beliefs can account for laws of logic, reason, uniformity of nature, or morality. Yet all of these immaterial entities are agreed to exist.
Only the Biblical God can account for things such as logic and reason.
Laws of logic are God’s standard for thinking. Since God is an unchanging, sovereign, immaterial Being, the laws of logic are abstract, universal, invariant entities. In other words, they are not made of matter—they apply everywhere and at all times. Laws of logic are contingent upon God’s unchanging nature. And they are necessary for logical reasoning. Thus, rational reasoning would be impossible without the biblical God.
Therefore, the atheist must be inconsistent and borrow from the Christian worldview (logic and reason) to argue against the Christian worldview.
As someone who used to be an atheist, I can tell you that the line:
"He believes that everything that exists is material"
is completely untrue. I, an every other atheist I knew, believed in things non-material. For starters, I believed in ghosts. I didn't believe there was a God in heaven, but I believed that people's ghosts could live on after their bodies.
I'm not arguing that atheists are consistent - in fact they are not. Perhaps if the line read "If he is a consistent atheist, he believes that everything that exists is material," that would be better suited.