Yes, perfect pitch is fairly rare at any age. I think, if you have it, then you have it from birth.
I come from a musically talented family. None of us are classically trained. I can't even read music, and have totally wasted my gift, but can hammer out pretty much any pop song on the piano, in a disgracefully amateur way.
Ds2 (aged 3) has perfect pitch. He never fails to get exactly the right notes when he sings theme tunes from his favourite tv shows. He gets quite upset when I sing them in the wrong key, because it's easier for my alto/tenor voice.
He also shows more than a few traits of asd. I've no idea if this is linked in any way.
Ds1 has pretty good relative pitch, but is no musical genius.
To be fair, I know two musical geniuses personally, and only one of them (the least commercially successful) has 'proper' perfect pitch. It's useful, but it isn't the most important thing. Perfect pitch is just a weird brain thing. It's possible that even really non musical people have it but don't know how to show it (they could maybe identify the correct frequency but lack the musical understanding to fit sounds together into a 'meaningful' whole that others find beautiful?)
Sounds like your ds is doing really well and enjoying music
and that's the main thing.