The methods I know of for teaching notation are:
a) positional method, where the child effectively plays only a very limited number of notes for quite some time, and learns to associate the written notes with the notes played by repetition of only that limited series of notes. Lots of the commercially available beginner books use this method - Alfred's is the one I'm most familiar with. With this method, the separate skills of playing the piano and learning to read, aren't divorced from each other.
b) staff method, where the child learns all the notes on the stave, usually by some sort of mnemonic (Every Good Boy Deserves Fun)
I'm not a teacher but a student
All I can say according to my experiences is that both methods are terrible and not effective
With the first one the kid gets so used to the limited series of notes that every other notes outside of this series if perceived as extraneous and there will always be a dangerous esitation when working on notes outside of the series
With the second method the kid gets used to mnemonics that lead to a sequence of thoughts, where we first have to remember the mnemonic before remembering the note names, that make the whole process highly distractful and slow, so that it becomes a real problem with fast speed pieces
The problem also is that what you learn a method at the begin is not easy to forget it later in favour of another method
So even though you son will grow on these methods as he approaches more advantaged pieces, the weakness of these methods will still be hunting him making the whole piano learning process harder than necessary
There's another method instead, more instinctive and meaningful, that I consider the best one
You just have to teach the kid how the "grand staff" is a visual rapresentation of the keyboard
In fact the lower the position of the notes on the grand staff, the lower the position of the notes in the keayboard and the higher
the position of the notes on the grand staff, the higher the position of the notes in the keayboard
I've created this image to illustrate what I mean:
(
you have to scroll or to save it on your computer to see the image in its whole length)

In this way within few weeks the kid will have learned how the grand staff mimics the piano keyboard and the position of every notes both on treble or bass cleff
By using this method the problem with the bass cleff and the dangerous tricks to memorize it are all useless as the kid doesn't think of the bass cleff as something separate from the treble cleff but just as the specular rapresentation of the lower part of the keyboard
The best way to learn reading notes is not by thinking of their names but learning reading by intervals instead
With this method the intervals are learned subtly and effortlessly without being spacifically taught
You can even teach the kid the position of the notes in the keyboard according to their position on the grand staff without the need to teach him the name of the notes (this can come later when all the position are learned)
Good luck
Daniel