It's VERY possible to play by ear that early - or earlier. I could do it on the piano by the time I was six. If he can sing on key, he's almost there; the only thing he has left to do is practice with the whistle enough that he can find the notes by instinct instead of hunting for them. At some point (for me, learning the whistle on my own but with years of piano and viola behind me, it was about two weeks in), he'll be able hear the note in his head and play it on the whistle immediately, without having to think about how many fingers to put down. If he wants to actively try to develop that, I'd suggest practicing scales and arpeggios a few minutes a day (not so long it gets boring) at the start to help him get a feel for where the notes are. Then start him on easy songs like Frere Jacques that have plenty of "climbing" and not so much "skipping" (that is, the notes tend to go up or down by one rather than jumping around). Down by the Sally Gardens, which is on the tutorial site OHGrandma recommended, is good for this too.
