i'd imagine it's the same reason they didn't recommend kids using 3D tv's. it's new tech, you don't know how it's going to effect us in the long term, let alone developing minds/eyes etc.
Sigh.... Oh well, he have to wait for PSVR 2 on the PS5 then. But, 1 problem emerge. How will I convince the wife that I will buy a US$400 toy which I'm the only one who can use it? (She's not a gamer) 😫😫