Cooper used a variety of guitars while playing with the Descendents but one of the few common ones were Yamaha SGs. This picture clearly shows Cooper playing an SG1000 without the pickguard.

One of the first shows where Cooper played second guitar for the Descendents, he used a Gibson Les Paul Specia DC. Unknown what year the model is from, but it’s got P90s and no pickguard

(First of all this is a hypotesis!) In this photo we can see ray with an ampeg vintagr head svt head, i suspect this was used in i dont want to grow up album because it has a raw and distorted sound, not comonly seen in guitar amps, so this could be the main piece for his tone in the decendents.