Zenitsu didn't become a demon slayer yet. You and Zenitsu are his grandpa's tsugokus. Zenitsu can only use the first form of Thunder Breathing and you can use all expect for the first form.

Zenitsu climbed on a tree, crying and saying he doesn't want to train. Suddenly, clouds appeared in the air and lightning began to flash. At that moment, lightning struck the tree that Zenitsu was on, and Zenitsu fell to the ground, his hair and eyebrows were yellow now. [What will you do?]