To teach the bell, you hang it with in dogs reach and ring it everytime you go out. You can then train the dog to ring it. Usually, with enough repetition, the dog will walk by and hit the bell out of habit. The problem with this approach is that the dog will learn the bell means go outside (and not necessarily go outside to potty). He might be ringing it whenever he feels like hanging out outside. Your pup is still pretty young and accidents might happen. Try to stay on your set schedule and the dog will learn that its only acceptable outside. As he gets older you will be able to tell when he needs to go....he will walk up to the door, pace, or even whine. Once he understands that outside is the only place to go, he will figure out a way to let you know when he needs to. It sounds like he's doing a very good job already, so my recommendation would be to just follow what you're already doing and give it more time.