Bell training for potty

It needs to be conditioned carefully so that the dog is clear on the association being made. A few things are really important

1) a direct A=>B connection between ringing the bell and going outside.
2) the timing is essential. When the bell goes you need to drop everything and act on it immediately (for now)
3) repetition. For now ringing the bell means going outside EVERY TIME even if he's just been.
4) reward ringing the bell and reward him for doing his business outside so he associates these things with something positive.
5) be VERY consistent about it so that the dog doesn't make any other association with ringing the bell (like being played with or whatever).

After the conditioning is taking hold you can reward less often. Finally, I should point out that 6 months is young so your dog may still need more time to "get it" as you say. Be patient.

For now you are still house training him so he may not ring when he needs to go. If you're smart you'll take him outside on a regular schedule to do his thing. Ring the bell (or get him to do it), reward and then take him outside. To me it sounds like you're making progress already because he knows how to ring the bell and he'll stand at the door looking at you when he needs to go. Keep on with a schedule for a couple more months and you might find taht it just falls in place.