I wish I could offer you more advice then you have in the article you posted.
Crosley we trained to ring a bell and we pretty much hung it on the door from the beginning. We rang it each time we went out and she picked it up pretty easily.
We tried to train Camden to ring the bells when we got her 1 1/2 yrs. ago since she wasn't that into the pads, but she didn't really pick up on it, until now!
It took her over a 1 1/2, but for some reason she just got it! I wouldn't say she does it 100% of the time, sometimes we walk by and she's standing by the door. Other times she will ring the bell 10 min. after she's been out!
She has also started to use the pads that Wrigley uses as well!
I would say stay consistent and he will eventually get it! Best of luck!