At age 4-5 asking them to decipher the hieroglyphics that western music notation presents may be a bit too much for him. Is he at the point yet where he can read simple children's books? I mean read as in the text, not tell the story from the pictures. I agree with miriamko. At this age perhaps what is best is to cultivate his curiosity with sound. Get him interested in the various timbres of instruments, feeling rhythm by banging on pots or drums, moving his body to music. The technicalities of playing piano can come much later when he is more comfortable with motor movements and reading ability. Look into music classes for very young children.
This may be off topic, but I'm curious as to your opinions: is reading music intrinsically harder than reading text? I learned to read text at around 3.5 years, but didn't start music until a few years later, so I don't really know how to compare.
I agree with m1469 about forming a special relationship with the instrument, and not just music. I don't like the bit about mentioning that the piano is "sad", and the "no friends" part, though.
Teach the student the song by rote, one part at time, one hand at a time. When they have already learned how to play it then you could explain how what they played relates to the music. You want to find a way to break down the music to simplify it for they by concentrating on different elements of the music.
One other side I tell my students parents is that practice is not always fun and it is a wrong way to look at learning piano. It is not always fun, it is hard work, that is the reality. If the kids are losing interest and not practicing that is generally an issue that you as a teacher have little control over.