How to train a lab corgi mix puppy to use the litter box?

Don't do this! Dogs are not cats and it's highly unlikely he'll manage to consistently use a litter tray in the same way a cat does (and even cats miss occasionally). Keep at it with taking him outside, every hour. If he doesn't go pretty much within 10 minutes of being out there, and stay with him, bring him in and confine him (crate recommended), leaving him for another 10 minutes or so, and repeating until he does go. If you try to use a litter box or those wretched pads, you are only telling him it's okay to do his biz indoors, which if only for hygiene reasons, it obviously isn't. Your place will start to smell. If you don't use a crate, confine him to the room where the door to the outside is, and stay with him. It should be easy to see him going to squat, and certainly before he poohs - when he does, get him outside, saying No!