A a is worse than silly. He's uncomprehending.
He actually right to to claim that water vapour is the most important greenhouse gas, but since atmospheric levels of water vapour equilibrate within about three weeks, there's no point in worrying about them.
CO2 levels takes a lot longer to settle down - roughly 800 years - and more CO2 in the air means that ocean surface temperatures are higher and the equilibrium level of water vapour in the atmosphere is higher and adds a bit more warming to the basic CO2 driven effect, It's a positive feedback, if not big enough to run away.