I've got that problem cos most say they can't read and write then.
My son knows his A-Z, can write all these letters unaided and also spell simple words unaided.
My husband who self learn piano (very very basic notes and songs) teaches him and he seems very interested, so I am keen to get him a proper teacher before we kill off his interest with our highly untrained teaching method.
I don't think it's to do with being able to read and write. I can't remember the reasons our piano teacher gave but I think it may have been to do with the size of a child's hands. Presumably if they're not big enough to stretch as they need to then they're not playing properly. This means they may have to relearn hand positions etc when they're older. I'm sure there were other reasons, but she was head of music at a secondary school for 20 or so years and has taught piano for much longer so I guess knows what she's talking about!
Can't you just let him mess about on the piano until he's a bit older?