In a weird way, he does have a point. For example, catalytic converters convert soot (carbon), CO (which, while not a greenhouse gas, competes for OH which leads to more CH4 in the atmosphere) and hydrocarbons into CO2 and H20. Remove the catalytic converters and less CO2 will be emitted. Of course the soot, carbon monoxide and hydrocarbons are worse for our health, especially on a short timescale.
It also leaves NOx out of the equation. NOx is a worse greenhouse gas (300x) then CO2 as well as being bad for you in small quantities and catalytic converters also change that into N2 and O to combine with the carbon to make even more CO2.
So in summary, get rid of catalytic converters and less CO2 will be emitted. In exchange we will have more greenhouse gases as well as more toxic gases.