In Too Deep - S5-E8
Plot hole: Carter is apparently just learning about Feynman's Day, which Allison explains as "Eureka's version of April Fool's Day." It's highly improbable that Carter could have lived in Eureka for five years and still be oblivious of this, especially given all of the hijinks that happen.
Chosen answer: He's still around, we've just not seen him lately, as scenes with Blake have been mainly at work, where Kevin wouldn't be.
Tailkinker ★