My family dog, a cockapoo, used to always attack my cat. In fact I have a few videos. He loved chewing on the poop in the litter box as well. After just telling him no, and rewarding him with treats when he stopped he grew out of it in a few months. E Collar would not be a good idea for such a young puppy. Just tell him 'No', and keep working in those training classes. Problem will most likely fix itself.
"Are you a dog person or cat person?"
"I am an animal person."
Proud owner of German/Aussie, Baylee.